Traded enum-based DSL for full callback-based DSL

Now, instead of passing an enum for mem/disk commits, we pass a function
pointer that can specify any behaviour.

This has the benefit of opening up the possibility to pass any sort of
commit logic to the committers, and unused logic can be garbage-collected
by the compiler if unused. The downside is that unfortunately compilers have
a harder time optimizing around functions pointers than enums, and
fitting the state into structs for the callbacks may be costly.

Author: geky