allow override of validate_proofs chunksize, fix float sats parsing, fix rewinds

[shuckc]

Fix a bug with rewinding to the height of a legitimate fork, which potentially causes a hang Check reserves input 'balance' values sum up correctly
Bitcoind memory use scales with chunksize, allow it to be overridden from cli. Fix an observed issue with float rounding during JSON parsing of amounts returned from scantxoutset.