Refactor some end-of-recovery code out of StartupXLOG().

Refactor some end-of-recovery code out of StartupXLOG().

Create a new function PerformRecoveryXLogAction() and move the
code which either writes an end-of-recovery record or requests a
checkpoint there.

Also create a new function CleanupAfterArchiveRecovery() to
perform a few tasks that we want to do after we've actually exited
archive recovery but before we start accepting new WAL writes.

More refactoring of this file is planned, but this commit is
just straightforward code movement to make StartupXLOG() a
little bit shorter and a little bit easier to understand.

Robert Haas and Amul Sul

Discussion: http://postgr.es/m/CAAJ_b97abMuq=470Wahun=aS1PHTSbStHtrjjPaD-C0YQ1AqVw@mail.gmail.com