Inquiries into the monitors, that is variables that
have a monitor attached to them, can be done on a
individual key, rather than getting the entire
lot and parsing them.
* common/Monitors.cpp, common/Monitors.h: Add dynamic variable
monitor functionality, allowing arbitrary variables to appear
in the monitors output.
* server/ServerRouting.cpp: Use the new monitor functionality for
variables prebviously pushed to monitors once.
* server/WorldRouter.cpp, server/WorldRouter.h: Add a count of
in-game entities. Expose it to the monitors.
* common/Monitors.cpp, common/Monitors.h: Add a new class to handle
storing key value pairs useful in monitoring the state of a server.
* server/HttpCache.cpp, server/HttpCache.h: Add a function argument
to allow the content type to be controlled in the header.
Expose the Monitors class under /monitors URL.
