Ensure no xid gaps during Hot Standby startup

In some cases with higher numbers of subtransactions
it was possible for us to incorrectly initialize
subtrans leading to complaints of missing pages.

Bug report by Sergey Konoplev
Analysis and fix by Andres Freund
Add a comment warning against use of pg_usleep() for long sleeps.

Follow-up to commit 873ab97219caabeb2f7b390268a4fe01e2b7518c, in which
I noted that WaitLatch was a better solution in the commit log message,
but neglected to add any documentation in the code.