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 
This is really nice - it almost entirely encapsulates the logic.

But why not go one step further?  Move this logic into `ScopedMetrics` - so this becomes just `ScopedMetrics::incrementFetches();`.