Switch IO manager to a mutable hashtable

This data structure (IntTable) provides a similar API to its predecessor
(IntMap), at half the number of lines in size.

When tested in isolation using criterion, IntTable is much faster than
IntMap: over 15x, according to my criterion benchmarks.

This translates into a measurable improvement when used in the IO manager:
using weighttp to benchmark acme-http under various configurations on two
32-core Linux servers connected by a 10gbE network, I see between a 3%
and 10% increase in requests served per second compared to IntMap.

Signed-off-by: Andreas Voellmy <andreas.voellmy@gmail.com>