blob: move heavyweight statvfs(3) call to periodic thread

This greatly speeds up common write path.
On my simple benchmarks it removes arount 7 system calls per write and
reduces time consumed by write-only tests by 50%.

This hovever increases race between statvfs call and actual write.
blob: allow eblob to init with very long paths

Problem was with stat_file being only 256 bytes long
blob: lock database on start

This will prevent multiple instances of libeblob from working with
single database.

Reported by: isharov@