Configure LoadBalancer with $wgSharedTables after WAN cache autodetection

Configure LoadBalancer with $wgSharedTables after WAN cache autodetection

Currently, $wgSharedTables - if configured - is injected into the LoadBalancer
before the WAN cache autodetection runs. As such, if $wgSharedTables is set
but $wgMainWANCache was not explicitly given, the LoadBalancer will force
a premature instantion of the WAN cache with an EmptyBagOStuff backend,
preventing the autodetection logic from setting a correct backend.
This change moves $wgSharedTables initialization after the WAN cache
autodetection to accunt for this scenario.

Change-Id: I59bb52f42905cda1c2aa47bc589a2f0b36b2a28f