Start fixing imap_read_headers() to account for MSN gaps. (see #3942)

Start fixing imap_read_headers() to account for MSN gaps. (see #3942)

Change the parameters to pass MSN instead of index, to make a bit
simpler.

Change header cache evaluation to look at the largest MSN retrieved
instead of ctx->msgcount for the next fetch start point.  This still
depends on the assumption that MSNs are retrieved in ascending order,
which needs to be fixed.

Simplify the header cache inner loop termination and memory cleanup
logic.  Fix a memory leak if a hole in the header cache occured.

Fix the header retrieval logic to take into account MSN gaps in the
results.  Loop only as long as we get IMAP_CMD_CONTINUE instead of
over a fixed count.  Simplify the inner loop termination and memory
cleanup logic too.

Simplify the "new mail while fetching" logic by creating a third outer
loop to handle re-fetches.

Fix msg_fetch_header() to return -2 if msg_parse_fetch() encounters a
corrupt FETCH response.  Previously it would pass on the rc of
msg_parse_fetch(), meaning it would return -1 even though the response
was corrupt.