The non-participation in procsignal was a problem for both changes in
master, e.g. parallelism not working for normal statements run in
walsender backends, and older branches, e.g. recovery conflicts and
catchup interrupts not working for logical decoding walsenders.

This commit thus replaces the previous WalSndXLogSendHandler with
procsignal_sigusr1_handler.  In branches since db0f6cad that can
lead to additional SetLatch calls, but that only rarely seems to make
a difference.

Author: Andres Freund
Reviewed-By: Michael Paquier
Discussion: https://postgr.es/m/20170421014030.fdzvvvbrz4nckrow@alap3.anarazel.de
Backpatch: 9.4, earlier commits don't seem to benefit sufficiently