prevent immature WAL streaming

Included 蔡梦娟 and Jakub Wartak because they've expressed interest on
this topic -- notably [2] ("Bug on update timing of walrcv->flushedUpto
variable").

As mentioned in the course of thread [1], we're missing a fix for
streaming replication to avoid sending records that the primary hasn't
fully flushed yet. This patch is a first attempt at fixing that problem
by retreating the LSN reported as FlushPtr whenever a segment is
registered, based on the understanding that if no registration exists
then the LogwrtResult.Flush pointer can be taken at face value; but if a
registration exists, then we have to stream only till the start LSN of
that registered entry.

This patch is probably incomplete. First, I'm not sure that logical
replication is affected by this problem. I think it isn't, because
logical replication will halt until the record can be read completely --
maybe I'm wrong and there is a way for things to go wrong with logical
replication as well. But also, I need to look at the other uses of
GetFlushRecPtr() and see if those need to change to the new function too
or they can remain what they are now.

I'd also like to have tests. That seems moderately hard, but if we had
WAL-molasses that could be used in walreceiver, it could be done. (It
sounds easier to write tests with a molasses-archive_command.)

[1] https://postgr.es/m/CBDDFA01-6E40-46BB-9F98-9340F4379505@amazon.com
[2] https://postgr.es/m/3f9c466d-d143-472c-a961-66406172af96.mengjuan.cmj@alibaba-inc.com

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