Fix assertion failure when updating full_page_writes for checkpointer.

Fix assertion failure when updating full_page_writes for checkpointer.

When the checkpointer receives a SIGHUP signal to update its configuration,
it may need to update the shared memory for full_page_writes and need to
write a WAL record for it.  Now, it is quite possible that the XLOG
machinery has not been initialized by that time and it will lead to
assertion failure while doing that.  Fix is to allow the initialization of
the XLOG machinery outside critical section.

This bug has been introduced by the commit 2c03216d83 which added the XLOG
machinery initialization in RecoveryInProgress code path.

Reported-by: Dilip Kumar
Author: Dilip Kumar
Reviewed-by: Michael Paquier and Amit Kapila
Backpatch-through: 9.5
Discussion: https://postgr.es/m/CAFiTN-u4BA8KXcQUWDPNgaKAjDXC=C2whnzBM8TAcv=stckYUw@mail.gmail.com