Fix signer id use in stackerdb

[jferrant]

In my massive PR, I aaccidentally conflated signer id and signer slot id. this did not break tests because we happened to have everything line up (odds are we ALWAYS will have everything line up since the reward set is used to buidl stackerdb, but this will make sure we do not rely on this)

[codecov]

Codecov Report

Attention: Patch coverage is 91.86047% with 14 lines in your changes are missing coverage. Please review.

Project coverage is 77.52%. Comparing base (911c3ff) to head (73fd5e0).

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4451      +/-   ##
==========================================
- Coverage   83.46%   77.52%   -5.94%     
==========================================
  Files         448      448              
  Lines      324308   324321      +13     
==========================================
- Hits       270676   251424   -19252     
- Misses      53632    72897   +19265     

Files	Coverage Δ
stacks-signer/src/client/mod.rs	99.09% <100.00%> (ø)
stacks-signer/src/client/stackerdb.rs	91.56% <100.00%> (-0.08%)	⬇️
stacks-signer/src/client/stacks_client.rs	83.03% <100.00%> (-3.71%)	⬇️
stacks-signer/src/config.rs	84.41% <ø> (ø)
stacks-signer/src/coordinator.rs	76.35% <100.00%> (ø)
stacks-signer/src/runloop.rs	90.50% <96.72%> (+2.36%)	⬆️
testnet/stacks-node/src/tests/signer.rs	66.98% <0.00%> (-26.41%)	⬇️
stacks-signer/src/signer.rs	73.66% <72.72%> (-0.14%)	⬇️

... and 173 files with indirect coverage changes

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[xoloki]

LGTM, one small question about a comment.

The base branch was changed.