Avoid sending large UID EXPUNGE|FETCH|STORE and APPEND commands.

UID EXPUNGE|FETCH|STORE commands are now split into multiple (sequential)
commands when their set representation exceeds 4096 bytes in size.  Without
splitting logic set representations could grow arbitrarily large, and
exceed the server's maximum command size.

This adds roundtrips which could be eliminated by pipelining, but it's
unlikely to make any difference in typical synchronization work.  While set
representations seem to remain small in practice, they might grow
significantly if many non-contiguous UIDs were flagged and/or expunged, and
later synchronized at once.

Furthermore, for MULTIAPPEND-capable servers, the number of messages is
limited to 128 per APPEND command (also subject to a combined literal size of
1MiB like before).

These numbers are currently not configurable.  They're intentionally lower
than Dovecot's default maximum command size (64k) in order to avoid a
deadlock situation after sending 8k-long commands under COMPRESS=DEFLATE:
https://dovecot.org/pipermail/dovecot/2019-November/117522.html .

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