imap: Introduced the mailbox_uidvalidity variable

Added the mailbox_uidvalidity variable to the per-connection structure
in preparation for checking the UIDVALIDITY mailbox attribute.
imap: Added support for parsing the UIDVALIDITY property

Added support for parsing the UIDVALIDITY property from the SELECT
response and storing it in the per-connection structure.
imap: Added verification of UIDVALIDITY mailbox attribute

Added support for checking the UIDVALIDITY, and aborting the request, if
it has been specified in the URL and the server response is different.