PITR: restore to a timestamp

[boddumanohar]

This PR implements restore functionality of point in time recovery.

The architecture diagram is as follows:

The backup is picked up automatically. For the given input timestamp, backup1 is picked up for recovery

----- T0 ----- T1 ----- T2 ------ T3 ------ T4 ----- CPIT
                     ^ PITR Target
                 ^ Restore Target                     ^ Additional WAL Segment Copy Source

[mxsrc]

Looks largely good, I have quite a few comments but they are all very localized changes.

[mxsrc]

Great, thanks for the changes. None of the changes should change any semantics, but I think it's still worth testing again to ensure everything works as intended.

[boddumanohar]

sure, yeah, I think it worth to redo the tests

[boddumanohar]

did a quick a test looks good and able to perform restore to a timestamp