Allow skipping/postponing the dn search

by refactoring the _LDAPQuery class:

  - slightly refactor _LDAPQuery.execute by splitting it into
    a new method _LDAPQuery.execute_cache, which does the real work
    of searching and caching, and a replacement _LDAPQuery.execute
    which will skip the call to execute_cache when filter_tmpl
    is empty.

  - directly call _LDAPQuery.execute_cache after entering the
    user-bind self.manager.connection() context manager
Escape login identifier before searching the entry.

This will avoid trivial DOS and ldap.FILTER_ERROR exceptions on
attempted logins by users sporting "funny" login names, like 'user*name'
or 'user(middle)name'.

This is a forward port of lmctv/pyramid_ldap@f3057446181106
to silence merge conflicts.