Fix for dinhviethoa/libetpan Github issue #68

* Modified to default to original behaviour if the server does not
implement the CONDSTORE capability.
* For servers that have CONDSTORE we use the mailimap_uid_search_modseq
function only if the search key type is one of
MAILIMAP_SEARCH_KEY_SEEN, MAILIMAP_SEARCH_KEY_ANSWERED,
MAILIMAP_SEARCH_KEY_FLAGGED, MAILIMAP_SEARCH_KEY_DELETED or
MAILIMAP_SEARCH_KEY_DRAFT and generate a new search key by means of
mailimap_search_key_new_modseq.