Add msn_index and max_msn to find and check boundaries by MSN. (see #3942)

Add msn_index and max_msn to find and check boundaries by MSN. (see #3942)

Since there can be gaps in MSNs, the largest MSN in the context is not
necessarily ctx->msgcount.

Use max_msn instead of ctx->msgcount for:
  - the starting MSN of new mail header fetching
  - boundary checking in fetch, expunge, and other places

Use msn_index to efficiently look up headers by MSN.  This makes the
expunge code slightly more efficient.  It also makes FETCH handling, and
duplicate FETCH FLAG handling efficient.