[server] implement dynamic database pooler Opening too much database connection has a cost at startup and also PostgreSQL as a maximum number of connection (100 by default). This get worse when starting multiple wsgi processes, since each process has its own database pool. Instead of opening `connections-pool-size` connections to the database at startup, just open one and open more only when needed.

[server] use a LifoQueue in _CnxSetPool In postgresql, some cache is attached to the connection. Using a LifoQueue (last-in, first-out) makes a few connections to get the most load which give best performance.

[server] extract creating a new cnxset in a _new_cnxset() helper So we can move logic specific to _CnxSetPool here.

[server] avoid a possible race condition on _CnxSetPool.close() The pool could become empty between time to check and time to use.

[server] extract "no pooler" CnxSet class to a _BaseCnxSet class So we get rid of "if self._queue is None" in each method of _CnxSetPool Also add helper get_cnxset(source, size) to instantiate the correct pooler class.

[server] prevent returning closed cursor to the database pool In since c8c6ad8 init_repository use repo.internal_cnx() instead of repo.system_source.get_connection() so it use the pool and we should not close cursors from the pool before returning it back. Otherwise we may have "connection already closed" error. This bug only trigger when connection-pool-size = 1. Since we are moving to use a dynamic pooler we need to get this fixed. This does not occur with sqlite since the connection wrapper instantiate new cursor everytime, but this occur with other databases.

[rql] Store selected variables for RQL select queries in ResultSet (#17218476) By storing the name of the selected variables for RQL select queries in the ResultSet (within the "variables" attribute), the information can be passed down to specific protocols, e.g. rqlio that may wish to pass is down further to clients. In turn, clients can then choose to present the results of RQL select queries as symbolic bindings using the names used in the query's projection, instead of ordinal arrays.

[debugtoolbar] Make SQL and RQL tables sortable It's usefull to sort query by duration for instance.

[debugtoolbar] Do not render CW controller panel when no controller got collected This can happen on pure pyramid applications for instance.

[doc] Fix bugs in code examples