Allow \ to escape wildcards in NXQL LIKE

We will add the possibility to escape the _ and % wildcards using a \ in a LIKE query:

SELECT ... WHERE dc:title LIKE 'foo\_'

This will only match foo_ but not for instance foo1. Without the \ it matches foo_ and foo1.

This ticket will also streamline the semantics of _ and % and their escaping with \ for all backends using NXQL queries: VCS, MongoDB, Elasticsearch.

Note that to match an explicit \ it has to be double-escaped, once for string-escaping, and once for LIKE-escaping, so you must write:

SELECT ... WHERE dc:title LIKE 'foo\\\\'

to be able to match foo\.