[req] New method: RequestSessionBase.find(). This method does what find_entities and find_one_entity did, except it returns the resultset itself. In addition, it accepts 'reverse_' arguments and check that the relations actually exists on the entity before executing the query. Also, reimplement find_one_entity and find_entity based on the new function so they benefit from the more complete implementation, and deprecate them. Note: List of values in kwargs are NOT supported in this initial implementation. Closes #3361290

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances