Using an Assert to check the validity of incoming messages is an
extremely poor decision.  In a debug build, it should not be that easy
for a broken or malicious remote client to crash the logrep worker.
The consequences could be even worse in non-debug builds, which will
fail to make such checks at all, leading to who-knows-what misbehavior.
Hence, promote every Assert that could possibly be triggered by wrong
or out-of-order replication messages to a full test-and-ereport.

To avoid bloating the set of messages the translation team has to cope
with, establish a policy that replication protocol violation error
reports don't need to be translated.  Hence, all the new messages here
use errmsg_internal().  A couple of old messages are changed likewise
for consistency.

Along the way, fix some non-idiomatic or outright wrong uses of
hash_search().

Most of these mistakes are new with the "streaming replication"
patch (commit 46482432), but a couple go back a long way.
Back-patch as appropriate.

Discussion: https://postgr.es/m/1719083.1623351052@sss.pgh.pa.us