Add RPC metrics (RIPD-705)

Add metrics to record the number of RPC requests received. Record the number of
node store fetches performed per request. Additionally record the byte size of
each request response and measure the response time of each request in
milliseconds.

A new class, ScopedMetrics, uses the Boost Thead Local Storage mechanism to
efficiently record NodeStore metrics within the same thread.

Reviewers: @vinniefalco , @rec 
ScopedMetric is a simple class that encapsulates the Boost Thread Local Storage mechanism and also stores the thread specific data. Its a very convenient and efficient way of sharing memory from different parts of the project within the context of a thread. In this particular case ServeHandlerImp or WSConnection and DatabaseImp.