api-thread-contention TODOs

currently working on a refactoring ndbapi for mysql-cluster, in order to decrease
mutex contention allowing to better utilize many threads on multi-core machines.
while doing so, i implement a "vanilla" ndbapi-backend which lacks features
current in there.

once this is working, i'll start adding optimizations...

reintroduce:
- zero copy receive
- optimizations for single threaded performance (less ctx switches)

introduce:
- split mutex(es)
- zero copy send (or actually 1-copy instead of 2, but zero sounds better)