Update some of client to v0.3 spec

[0xKiwi]

Part of #1542

This updates some segments of the client to the v0.3 spec, I was doing the validator spec changes and saw there were other changes needed.

Notable changes:

• EpochLength -> SlotsPerEpoch
• slotDuration -> secondsPerSlot (only a few)
• CurrentBoundaryAttestations -> CurrentEpochBoundaryAttestations (let me know if this one isn't needed, I figured it should match the spec for consistency even if it's a little long)
• PrevBoundaryAttestations -> PrevEpochBoundaryAttestations (same as above)
• Some instances were still AttestationInfo instead of the current AttestationData

Doc changes:

• ShardNumber -> Shard (only doc changes)
• EpochNumber -> Epoch (only doc changes)
• previous_shuffling_seed -> previous_epoch_seed (only doc changes)
• previous_shuffling_seed -> previous_epoch_seed (only doc changes)

Also handles: 629, and 630, as requested in #1542 which are minimal changes.

[codecov]

Codecov Report

Merging #1733 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1733   +/-   ##
=======================================
  Coverage   72.06%   72.06%           
=======================================
  Files         102      102           
  Lines        7252     7252           
=======================================
  Hits         5226     5226           
  Misses       1557     1557           
  Partials      469      469

[nisdas]

lgtm