Pool of Threads dispatching queries to DB servers

It can be done with the following steps:

1) Create interface for Dispatcher class, make one more type of plugin and move current implementation to plugin;

2) Use shared_ptr to queries queue instead of queries queue itself in DBThread class. The current implementation will create new instance of queue for each DBThread instance but thread pool will have one queue for all threads;

3) Remove pointer to DBThreadState from DBThread class. As we have pool of connection threads we should not store DB thread state in connection class;

4) Create new dispatcher plugin. Let's name it as ConnectionsPoolDispatcher. There will be --connections-pool-thread-no command line option wich allows to set threads number in threads pool;

a) create N DBThread instances with the pointers to the one queue of QueryEntries objects;

b) keep pointer to DBConnectionState in the descendants of QueryEntry class;

c) tcpdump plugin becomes responsible for managing it connection states;

d) Dispatcher::dispatch() just adds new QueryEntry to the end of queries queue;

e) Dispatcher::finish_and_wait() does nothing;

f) Dispatcher::finish_all_and_wait() pushes "finish" query entry to the queue and join to all spawned threads. If any thread pops such entry it invokes tbb::concurrent_bounded_queue::abort() to finish other threads.