Compact asynchronous pooling

[vanrein]

We're using a macro package, uthash.h, which expands to quite some code.

1. We use a few routines twice. Perhaps turn them into functions.
2. Internal macros that come back: HASH_VALUE, and probably a few of the _FIND_ operations; we can put them in a function and redefine the macro to that function right below its definition.

A matter of trying.

[vanrein]

Another consideration: some applications might prefer utlist.h instead of uthash.h, with smaller storage but slower responses. This would not be ABI-compatible, but a local version (perhaps with other names, or not installable, or some kind of assert() to verify in time) might be considered.