Invent WALG_PREFETCH_DIR

[x4m]

This will help to resolve issue #904

@CyberDem0n please take a look. Does it help with #904?

[CyberDem0n]

Yes, it would be nice to have it, but we can do better than that.
During normal WAL replay postgres passing pg_wal/RECOVERYXLOG as the %p argument, but pg_rewind calls it with pg_wal/%f.
It already gives us enough information for disabling prefetch if WALG_PREFETCH_DIR is not set or if it known to be inside the PGDATA.

I ended up wrapping wal-g into a bash script that checks if %p ends with /%f and sets WALG_DOWNLOAD_CONCURRENCY to 1.

[x4m]

Uhm....soooooo hacky...

[CyberDem0n]

Uhm....soooooo hacky...

Yes, it is, but also it reducing chances of user error down to absolute zero.