* speed up state/block loading
When loading blocks and states from db/era, we currently redundantly
check their CRC32 - for a state, this costs 50ms of loading time
presently (110mb uncompressed size) on a decent laptop.
* remove `maxDecompressedDbRecordSize` - not actually used on recent
data since we store the framed format - also, we're in luck: we blew
past the limit quite some time ago
* fix obsolete exception-based error checking
* avoid `zeroMem` when reading from era store
see https://github.com/status-im/nim-snappy/pull/22 for benchmarks
* bump snappy
e8379389e7