Allow passing an adapter to Gitlab::LDAP::Person
Add Gitlab::LDAP::Adapter.open

This new method is based on Net::LDAP.open, which reuses a single LDAP
connection.
Add Gitlab::LDAP::Access.open

This new method wraps Gitlab::LDAP::Adapter.open to enable connection
reuse.
Open/close LDAP in ApplicationController

By opening the LDAP connection at the controller level we can reuse it
for all LDAP queries during the request.
Merge branch 'ldap_connections'

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Conflicts:
	CHANGELOG