Drop unused cbdata.h definitions and re-document

Drop unused cbdata.h definitions and re-document

Remove the now unused cbdataFree, cbdataAlloc, CBDATA_TYPE,
CBDATA_INIT_TYPE, CBDATA_INIT_TYPE_FREECB symbols.

Re-write CBDATA documentation to reflect the current available
API symbols, their usage, and mechanisms that should be used
instead of CBDATA such as AsyncJob/Call and RefCount.

Along with some doxygen polishing to meet currently agreed
style for how to document major code features.

Make generic_cbdata::data a private member. The constructor
and unwrap() operator provide all necessary public API.

Replace store_client.cc use of cbdataInternalLock/Unlock with
a CbcPointer<> smart pointer equivalent. The use remains an
abuse of CBDATA, just no longer directly referencing the
internal API functions.