Speed up altair block processing 2x (#3115)
* Speed up altair block processing >2x
Like #3089, this PR drastially speeds up historical REST queries and
other long state replays.
* cache sync committee validator indices
* use ~80mb less memory for validator pubkey mappings
* batch-verify sync aggregate signature (fixes #2985)
* document sync committee hack with head block vs sync message block
* add batch signature verification failure tests
Before:
```
../env.sh nim c -d:release -r ncli_db --db:mainnet_0/db bench --start-slot:-1000
All time are ms
Average, StdDev, Min, Max, Samples, Test
Validation is turned off meaning that no BLS operations are performed
5830.675, 0.000, 5830.675, 5830.675, 1, Initialize DB
0.481, 1.878, 0.215, 59.167, 981, Load block from database
8422.566, 0.000, 8422.566, 8422.566, 1, Load state from database
6.996, 1.678, 0.042, 14.385, 969, Advance slot, non-epoch
93.217, 8.318, 84.192, 122.209, 32, Advance slot, epoch
20.513, 23.665, 11.510, 201.561, 981, Apply block, no slot processing
0.000, 0.000, 0.000, 0.000, 0, Database load
0.000, 0.000, 0.000, 0.000, 0, Database store
```
After:
```
7081.422, 0.000, 7081.422, 7081.422, 1, Initialize DB
0.553, 2.122, 0.175, 66.692, 981, Load block from database
5439.446, 0.000, 5439.446, 5439.446, 1, Load state from database
6.829, 1.575, 0.043, 12.156, 969, Advance slot, non-epoch
94.716, 2.749, 88.395, 100.026, 32, Advance slot, epoch
11.636, 23.766, 4.889, 205.250, 981, Apply block, no slot processing
0.000, 0.000, 0.000, 0.000, 0, Database load
0.000, 0.000, 0.000, 0.000, 0, Database store
```
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