Server: Only send node removal/addition to client if client's player is near it. Otherwise set the modified blocks unsent.

--HG--
rename : data/tool_stpick.png => data/tool_stonepick.png
rename : data/tool_wpick.png => data/tool_woodpick.png

Made a proper queued thread to client for handling some block mesh updates. Also made client mutex-free to allow easier adding of new stuff.

Modified block mesh generation to have clearer input and output. Instead of being a messy object oriented cludge, it now is a messy cludge with separate parameters.