Make walsender more responsive.

Per testing by Andres Freund, this improves replication performance
and reduces replication latency and latency jitter.  I was a bit
concerned about moving more work into XLogInsert, but testing seems
to show that it's not a problem in practice.

Along the way, improve comments for WaitLatchOrSocket.

Andres Freund.  Review and stylistic cleanup by me.

src/backend/port/unix_latch.c

@@ -418,6 +418,9 @@ WaitLatchOrSocket(volatile Latch *latch, int wakeEvents, pgsocket sock,
  * NB: when calling this in a signal handler, be sure to save and restore
  * errno around it.  (That's standard practice in most signal handlers, of
  * course, but we used to omit it in handlers that only set a flag.)
+ *
+ * NB: this function is called from critical sections and signal handlers so
+ * throwing an error is not a good idea.
  */
 void
 SetLatch(volatile Latch *latch)