services/mounttable/mounttablelib: Add node counter

This change adds a counter to keep track of the number of nodes in the
mounttable. This counter is exported in the stats as
mounttable/num-nodes.

Partly addresses https://github.com/veyron/release-issues/issues/1896

MultiPart: 1/3

Change-Id: I9844b18e9be85a381a2a3bed9c431d5499bbea4b
15 files changed