Unrefs objects after calls to g_async_result_get_source_object()
Fixes error in write_request_headers() of EvdHttpConnection
Removes connection ref increase in pool_finish_request() of EvdConnectionPool

This is causing leaks in connections create by pools.
Removes unnecessary referencing of the socket by EvdConnection

A reference to the socket is already maintained by EvdSocketInputStream and
EvdSocketOutputStream.