Limit the number of OS threads spawned by go-runtime

Currently, there is no limit on the number of OS threads spawned by a go-program. These OS threads once spawned will not exit. They just wait for new go-routines to be scheduled on them.

On a system where there is a limit on max. number of threads that can be spawned (E.g., on CentOS 6, the maximum couchbase user processes are limited to 1024), this can become a problem as indexer process can spawn large number of threads and the OS will not allow any other couchbase process to spawn threads. If other couchbase processes, tries to spawn a thread, they would crash as the limit is exhausted.

The idea behind this improvement is to track the users encountering this issue.