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 
@rec Nothing wrong with new code using the new convention. Its up to the developer if they want to change the old occurrences. Eventually, I'm sure they will all be converted. There's no rush though. Consistency here adds no real value.