Fixes and improvements to IntObjectHashMap

- Rewrite with linear probing, no state array, compaction at cleanup
- Optimize keys() and values() to not use reflection
- Optimize hashCode() and equals() for efficient iteration
- Fixed equals() to not return true for equals(null)
- Optimize iterator to not allocate new Entry at each next()
- Added toString()
- Added some new unit tests
@normanmaurer fixed and pushed. Also reported an issue for my full-build problem (this is on Goobuntu Trusty, and I don't normally compile native code so maybe something else missing in my env.) But I have ran most tests, everything before that failure works including http2.