integrate thread pool with server packet processing implementation
refactor server code to use thread pool packet processing
this issue will handle the integration of the packet thread pool with the existing server code. it will disable MT by default. it also adds a command line parameter '-N' which can be used to explicitly specify the number of threads. currently this parameter does nothing. #893 will add configuration to the server config file and the possibility to enable MT by specifying the thread count on command line.
please review !665 (merged) which is related to this issue.