merge: upstream fixes (mutt/default)

 * Move the IMAP msn field to IMAP_HEADER_DATA. (see #3942)
 * Fix imap expunge to match msn and fix index. (see #3942)
 * Fix cmd_parse_fetch() to match against MSN. (see #3942)
 * Start fixing imap_read_headers() to account for MSN gaps. (see #3942)
 * Add msn_index and max_msn to find and check boundaries by MSN. (see #3942)
 * Properly adjust fetch ranges when handling new mail. (see #3942)
 * Small imap fetch fixes. (see #3942)
 * Don't abort header cache evaluation when there is a hole. (see #3942)
 * Fix mfc overflow check and uninitialized variable.
 * Fix potential segv if mx_open_mailbox is passed an empty string. (closes #3945)
 * Don't clean up idata when closing an open-append mailbox.
 * Don't clean up msn idata when closing an open-append mailbox. (see #3942)
 * Fix memory leak when closing mailbox and using the sidebar.
 * Change imap body cache cleanup to use the uid_hash.