Boost influence on engine compile time

[aryjczyk]

net::GameState uses boost::multi_array internally, but because of header inclusions boost::multi_array is also included in compilation units of all engines, which can have negative effect on their critical compilation time. After whole system, including engine compilation on the fly, is up and running boosts impact needs to be measured and issue fixed, if it's significant. Possible fixes: use pimpl idiom in net::GameState, change internal board representation, forward declare boost::multi_map(some function definitions may need to be moved from header file).