Merged evtp.h and threadpool.[ch] into ylib + use a global thread pool

This is mostly just a code cleanup. The new evtp.[ch] may be slightly
more efficient because it avoids a few extra allocations and has been
modified to make use of some libev features. It is also a bit smaller.

Additionally, this thread pool should make clean shutdown easier to
implement, which is a goal I'll try to work on before adding support for
storage.