avoid packing attestations from other forks

[etan-status]

When there is heavy forking, proposals may get missed due to including
attestations from different forks that later fail verification.
Checking attestation signatures when building blocks should fix this.

[etan-status]

DBG 2022-07-20 13:53:00.015+00:00 Packed attestations for block              topics="attpool" newBlockSlot=240 packingDur=2ms566us314ns totalCandidates=69 attestations=57
ERR 2022-07-20 13:53:00.019+00:00 Cannot create block for proposal           topics="beacval" slot=240 head=0d13c2df:238 error="indexed attestation: signature verification failure"

[tersec]

It would be useful to have some measurements performancewise of how much this matters. Hitting proposer boosting is useful.

[etan-status]

without this:

Running state_sim --validators=8000 --slots=160

Loaded genesis_mainnet_8000_1.2.0-rc.1.ssz...
Starting simulation...
:............................... slot: 32 epoch: 1
:............................... slot: 64 epoch: 2
:............................... slot: 96 epoch: 3
:............................... slot: 128 epoch: 4
:............................... slot: 160 epoch: 5
:Done!
Validators: 8000, epoch length: 32
Validators per attestation (mean): 250.0
All time are ms
     Average,       StdDev,          Min,          Max,      Samples,         Test
       0.827,        0.162,        0.727,        2.442,          155, Process non-epoch slot with block
       3.398,        0.689,        2.268,        4.110,            5, Process epoch slot with block
       0.018,        0.005,        0.008,        0.051,          160, Tree-hash block
      84.659,        3.710,       78.661,       98.758,          160, Combine committee attestations

Running block_sim --validators=8000 --slots=160

Loaded genesis_mainnet_8000_1.2.0-rc.1.ssz...
Starting simulation...
................................ slot: 32 epoch: 1
................................ slot: 64 epoch: 2
................................ slot: 96 epoch: 3
................................ slot: 128 epoch: 4
................................ slot: 160 epoch: 5
Done!
Validators: 8000, epoch length: 32
Validators per attestation (mean): 0.0
All time are ms
     Average,       StdDev,          Min,          Max,      Samples,         Test
      16.805,        7.705,       12.502,      109.302,          155, Process non-epoch slot with block
     162.210,      193.915,       28.946,      461.087,            5, Process epoch slot with block
       0.026,        0.005,        0.013,        0.052,          160, Tree-hash block
       0.335,        0.056,        0.294,        0.688,          160, Sign block
      72.598,        4.716,       64.517,       92.929,          160, Have committee attest to block
     272.536,      135.019,        0.000,      403.813,          160, Produce sync committee actions
      54.321,        0.000,       54.321,       54.321,            1, Replay all produced blocks

with this:

Running state_sim --validators=8000 --slots=160

Loaded genesis_mainnet_8000_1.2.0-rc.1.ssz...
Starting simulation...
:............................... slot: 32 epoch: 1
:............................... slot: 64 epoch: 2
:............................... slot: 96 epoch: 3
:............................... slot: 128 epoch: 4
:............................... slot: 160 epoch: 5
:Done!
Validators: 8000, epoch length: 32
Validators per attestation (mean): 250.0
All time are ms
     Average,       StdDev,          Min,          Max,      Samples,         Test
       0.823,        0.155,        0.727,        2.330,          155, Process non-epoch slot with block
       3.253,        0.454,        2.448,        3.528,            5, Process epoch slot with block
       0.017,        0.002,        0.009,        0.038,          160, Tree-hash block
      81.231,        2.815,       78.258,       96.779,          160, Combine committee attestations

Running block_sim --validators=8000 --slots=160

Loaded genesis_mainnet_8000_1.2.0-rc.1.ssz...
Starting simulation...
................................ slot: 32 epoch: 1
................................ slot: 64 epoch: 2
................................ slot: 96 epoch: 3
................................ slot: 128 epoch: 4
................................ slot: 160 epoch: 5
Done!
Validators: 8000, epoch length: 32
Validators per attestation (mean): 0.0
All time are ms
     Average,       StdDev,          Min,          Max,      Samples,         Test
      41.949,        7.694,       17.563,      106.825,          155, Process non-epoch slot with block
     188.216,      186.439,       58.099,      467.096,            5, Process epoch slot with block
       0.028,        0.011,        0.013,        0.093,          160, Tree-hash block
       0.328,        0.052,        0.294,        0.655,          160, Sign block
      70.252,        2.788,       64.091,       78.638,          160, Have committee attest to block
     264.344,      130.261,        0.000,      355.739,          160, Produce sync committee actions
      54.099,        0.000,       54.099,       54.099,            1, Replay all produced blocks

Not sure if those benchmarks are the correct ones to measure this.
And it's true that in periods of heavy forking, it is quite questionable whether the extra rewards offset any penalties from potentially missing proposer boost cutoff.
A hybrid could be made that limits the time to collect attestations to an upper bound, but not sure how high that one should be set.

[github-actions]

Unit Test Results

       12 files  ±0       860 suites  ±0   1h 1m 39s ⏱️ - 12m 28s
  1 908 tests ±0    1 760 ✔️ ±0  148 💤 ±0  0 ❌ ±0 
10 349 runs  ±0  10 143 ✔️ ±0  206 💤 ±0  0 ❌ ±0 

Results for commit 338ba37. ± Comparison against base commit df66029.