Introduce connection pool (#2829)

Make it hold connection kwargs for local connections and all `NamedConnection` objects use them automatically.

Also get rid of redundant `ConfigHandler.local_connect_kwargs`.

On top of that we will introduce a dedicated connection for the REST API thread.

patroni/postgresql/bootstrap.py

@@ -176,7 +176,7 @@ class Bootstrap(object):
         """
         cmd = config.get('post_bootstrap') or config.get('post_init')
         if cmd:
-            r = self._postgresql.config.local_connect_kwargs
+            r = self._postgresql.connection_pool.conn_kwargs
             connstring = self._postgresql.config.format_dsn(r, True)
             if 'host' not in r:
                 # https://www.postgresql.org/docs/current/static/libpq-pgpass.html