nsqd/nsqlookupd: simplify protocol magic handling
nsqd/nsqlookupd: forceably close connections on invalid magic
nsqlookupd: improve reliability of tests
nsqd: fatal errors and responses

* SUB / IDENTIFY now have success responses
* all errors except for FIN / REQ commands are fatal
* invalid RDY count values are now fatal
* simplified set of error codes
* add context to error frames
update ChangeLog and upgrade notes
Merge pull request #133 from mreiferson/fatal_errors_133

nsqd: v2 protocol responses/fatal errors