nsqd: add /debug/pprof/{cmdline,symbol}

with these two endpoints you can directly profile a running nsqd:

    $ go tool pprof http://127.0.0.1:4151/debug/pprof/heap
    $ go tool pprof http://127.0.0.1:4151/debug/pprof/cpu
Merge pull request #248 from mreiferson/symbol_248

nsqd: support /debug/pprof/{cmdline,symbol}
nsqd: cpu profile should be .../profie to support direct profiling
nsqd: limit the amount we will read for HTTP publishing
reader: lookupd poll changes

* configurable jitter
* 60s default poll interval
* query one lookupd at a time
* handle properly reconnects w/ a pending connection
Merge pull request #252 from mreiferson/cpu_profile_endpoint_252

nsqd: use correct CPU profile endpoint
Merge pull request #251 from mreiferson/http_publish_limits_251

nsqd: better enforcement of body/msg limits in HTTP publish
Merge pull request #250 from jehiah/lookupd_poll_interval_250

reader: distribute lookupd requests
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