etcdhttp: perform validation of query parameters

Add basic input validation of all query parameters supported by
serveKeys. Also restructures etcdhttp a bit to better facilitate
testing.

Test coverage is slightly improved.
etcdhttp: add tests for ParseBool/ParseUint64 helpers
etcdserver: address style comments
etcdserver: switch to using etcd.Error
etcdserver: restructure to hopefully simplify

Obviate parseBool helper, define emptyReq locally to parseRequest,
have writeEvent return an error which gets logged
etcdhttp: use Form values over query parameters
simplify
Merge pull request #1011 from jonboulle/clean_http

Validate HTTP API user input