Keep parsoidService from ending a response too soon.

* Restore env.responseSent flag to prevent parsoidService from ending
  a response early. Previously, we used the res.headersSent flag as
  a proxy for the whole response having been sent. We would not end /
  send responses if headersSent was set to true. However, we discovered
  that the first call to res.write() will send headers, even if the
  entirety of the response has not yet been sent. Therefore, nothing
  was being transmitted after the first call to res.write().

* Small fixes to parserTests and Logger; parserTests now uses console.
  log, rather than env.log, to print out parserTest options.

Change-Id: I32526f3538c1dd10b6bacaa45a94f46b9d14d952