Rework ldap integration with ldap3

Use the pure-python ldap3 library, which eliminates the need for a
system dependency.

Offer both a `search` and `simple_bind` mode, for more sophisticated
ldap scenarios.
- `search` tries to find a matching DN within the `user_base` while
  employing the `user_filter`, then tries the bind when a single
  matching DN was found.
- `simple_bind` tries the bind against a specific DN by combining the
  localpart and `user_base`

Offer support for STARTTLS on a plain connection.

The configuration was changed to reflect these new possibilities.

Signed-off-by: Martin Weinelt <hexa@darmstadt.ccc.de>

tests/utils.py

@@ -56,6 +56,7 @@ def setup_test_homeserver(name="test", datastore=None, config=None, **kargs):
 
     config.use_frozen_dicts = True
     config.database_config = {"name": "sqlite3"}
+    config.ldap_enabled = False
 
     if "clock" not in kargs:
         kargs["clock"] = MockClock()