More than doubles the speed of verifying already-cached signatures
that use compressed pubkeys:
Before: ~200 microseconds
After: ~80 microseconds
(no caching at all: ~3,300 microseconds per signature)
Also encapsulates the signature cache code in a class
and fixes a signed/unsigned comparison warning.
Create a maximum-10MB signature verification result cache.
This should almost double the number of transactions that
can be processed on a given CPU, because before this change
ECDSA signatures were verified when transactions were added
to the memory pool and then again when they appeared in
a block.
