go-raw-mongodb clean up

[andregmoeller]

This is not a pull request, which improves the performance. It is meant to clean up the go-raw-mongodb implementation.
I did the following changes:

• Fix warning concerning the content-type in fortuneHandler
• Add additional error handling to fortuneHandler
• make sure that the identity column is _id (see http://frameworkbenchmarks.readthedocs.org/en/latest/Project-Information/Framework-Tests/: ...except for MongoDB, wherein the identity column is _id, with the leading underscore)
• removed runtime.GOMAXPROCS(runtime.NumCPU()) (see https://golang.org/doc/go1.5: By default, Go programs run with GOMAXPROCS set to the number of cores available; in prior releases it defaulted to 1.)
• removed unnecessary if/else/return (log.Fatal/log.Fatalf is equivalent to Print()/Printf() followed by a call to os.Exit(1).)
• simplified the dbHandler and queriesHandler
• In my opinion the original implementation of the Database update test violated requirement numbers 4, 5 and 6 - in my opinion the original code didn't read the original randomNumber field from the database:
  • The request handler must retrieve a set of World objects, equal in count to the queries parameter, from the World database table.
  • Each row must be selected randomly using one query in the same fashion as the single database query test (Test Benchmarking tool suggestion. #2 above). As with the read-only multiple-query test type (Update and improve Compojure #3 above), use of IN clauses or similar means to consolidate multiple queries into one operation is not permitted.
  • At least the randomNumber field must be read from the database result set.

[ssmith-techempower]

Looks good to me, merging in.