retention option: wal-depth

[gsmolk]

It would be nice to have an ability to govern the amount of WAL keeped by retention measured by a number of backups.

A ---> --A1 ---> A2 ---> A3 --->    timeline 1

A - FULL backup
A{1,3} - incremental backups.

If wal-depth is set to 1, then keep only WAL >= START LSN of A3.
If wal-depth is set to 2, then keep only WAL >= START LSN of A2.

[gsmolk]

New option --wal-depth=number - number of latest valid backups in timeline that can be used to perform PITR.

[alubennikova]

I slightly updated code comments.
It seems that the case of retention with multiple timelines (each of which contains several backups) must be improved. I left the corresponding TODO comment.

This issue lacks help.c and Documentaion.md update.

[gsmolk]

It seems that the case of retention with multiple timelines (each of which contains several backups) must be improved.

Done
TODO: Refactoring, documentation, comments improvements, more tests

[gsmolk]

Merged.