GUC-ify walsender MAX_SEND_SIZE constant

Hello all,

Following the threads here
<https://www.postgresql.org/message-id/flat/CAFWczPvi_5FWH%2BJTqkWbi%2Bw83hy%3DMYg%3D2hKK0%3DJZBe9%3DhTpE4w%40mail.gmail.com>
and there <https://commitfest.postgresql.org/13/958/>, I decided to submit
this patch.

Following is the description which is also written in the commit message:
MAX_SEND_SIZE parameter was used in WALSender to limit maximum size of
a WAL data packet sent to a WALReceiver during replication. Although
its default value (128kB) was a reasonable value, it was written in
2010. Since the hardwares have changed a lot since then, a PostgreSQL
user might need to customize this value.
For example, if a database's disk has a high bandwidth and a high
latency at the same time, it makes more sense to read bigger chunks of
data from disk in each iteration. One example of such disks is a remote
disk. (e.g. an RBD volume)
However, this value does not need to be larger than wal_segment_size,
thus its checker function returns false if a larger value is set for
this.

This is my first patch. So, I hope I haven't done something wrong. :'D

Best regards
Majid