Rename EnginioClientBase to EnginioClientConnection.

The "Base" name was meaningless. EnginioClientConnection is keeping
information about backend connection.

[ChangeLog][API] Renamed EnginioClientBase to EnginioClientConnection

Change-Id: Ide98c67702c2edd3ba2485d10f619917671834b4
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>