[server.querier] construct ExecutionPlan from schema since querier is not needed

[server.querier] remove useless attribute Querier._planner

[server.querier] remove useless indirection Querier.rqlannotator

[server.querier] remove useless indirections ExecutionPlan.{syssource,rqlhelper,schema} Simpler is better.

[server.rqlannotation] rename SQLGenAnnotator to RQLAnnotator This class is in charge of annotating the RQL syntax tree with various bits of information, like (in)variance, use of full-text-index, etc. It is a needed step before the generation of SQL, but does not touch SQL directly. Hence RQLAnnotator seems a better name.

[server] drop system_source_cnx for a simpler internal API system_source_cnx() can be replaced with source_cnx(). If system_source_cnx() is called with dbms_system_base=False, it is strictly equivalent to calling source_cnx() with the same source, privileges and interactivity. If system_source_cnx() is called with dbms_system_base=True, it is equivalent to calling source_cnx() with the system database and the same source, privileges and interactivity. No use of the default value for 'special_privs' was found in the code.

[server/sources/native] refactor to remove useless injection

[cwvreg] rename CWRegistryStore.solutions to compute_var_types solutions() seems an obscure name to me. Since this method computes the types of the variables of the RQL query string, let us rename it to compute_var_types.

[doc] replace python with python3 in shell commands

[adapters] move class EntityAdapter from cubicweb.view to cubicweb.entity The adapter mechanism is not specific to the use of views. Moving the class the cubicweb.entity makes this clearer.