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 (closed) 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.