sha256 minor tidying.
increase credits/debits checksum to 8 bytes up from 4 to protect against collisions.
just some debug logging to poller and indexer. might be useful in the future.
pool/indexer detailed debug logging isn't needed in main code though.

This reverts commit 11fd8968752b0f60b0893ee6f273ab946f5ab4f7.
Merge branch 'master' of https://github.com/spesmilo/libbitcoin