Don't use Asserts to check for violations of replication protocol.

Don't use Asserts to check for violations of replication protocol.

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 464824323), but a couple go back a long way.
Back-patch as appropriate.

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