[rset] Use RQL cache in syntax_tree method This avoid several unnecessary parsings per HTTP request, since in most case rset's rql is in the rql cache (since it has just been executed), while yapps based parsing is known to be slow.

[repo] Introduce a clear_caches method on the Querier class that is called from repository's clear_caches.

[repo] Consistent API for cache clearing clear_caches, reset_caches, clear_eid_caches are now all named 'clear_caches' with optional eids/etypes arguments to clear eid specific cache entry,

[repo] Extract rql cache handling to a dedicated class