Substantive changes:
* Remove the parameters globalKeyLB and localKeyLB, not needed in
production, probably not used anywhere.
* Add a "cluster" parameter, for production.
* Don't call markServerDown() in LoadBalancer mode.
Style changes:
* Document config parameters separately from constructor parameters, to
clarify the role of the wiring normalisation.
* Inject a load balancer from ObjectCache to SqlBagOStuff, using a
closure to defer construction.
* Clarify the split between LoadBalancer mode and server array mode by
introducing a useLB property. Use it instead of special shard index
values and array counts.
* multiPrimaryModeType wasn't needed anywhere and wasn't available in
the constructor, so it just became a boolean multiPrimaryMode.
* Add a test covering most of the server array (!useLB) branches.
Bug: T212129
Change-Id: Ib097082efb40b514a29a42286dc362f2e3743ee4
