IMAP capabilities and selected folder are IMAP session,
not IMAP client property.
Moving most operations into IMAP session structure
removes the need to constantly check whether IMAP session exists
and reduces number of invalid states, e.g. when a folder is selected but
there is no connection.
Capabilities are determined immediately after logging in,
so there is no need for `capabilities_determined` flag anymore.
Capabilities of the server are always known if there is a session.
`should_reconnect` flag and `disconnect()` function are removed: we
drop the session on error. Even though RFC 3501 says that a client
SHOULD NOT close the connection without a LOGOUT, it is more reliable
to always just drop the connection, especially after an error.
