update comment
tweak server startup waiting in test
refine param hash

- same nested param asetting behavior as rack.
- raise TypeError instead of RuntimeError when key check fails.
go back to non-sharing-key-stack param parsing
fix ThreadError

just stop forked workers in reloader and leave threaded impls alone
fix tests

- turn private method _aref public
- use `send` for calling private method `nested_aset`