client: Don't lose trace_id at timeout
client: Recover files on read also for -EILSEQ

This error means "checksum check failed", so data is corrupted
client: Fixed checkers at merge_indexes
client: Throw -EPROTO if we connected to wrong addr
server: Fixed io queue limits

We should also consider backends' own pools
Merge pull request #504 from euroelessar/master

Fixed several errors