common: Use metric_t internally

common: Use metric_t internally

This is a WIP

This commit is where the major transform takes palce; the internal
representation is changed, and the read plugins use that indirectly
when they submit a value list transparently.

The baromerter read plugin has been converted to using metric_t
representation natively.

The write plugins have to be adapted to use these new single data
source metrics.

Signed-off-by: Manoj Srivastava <srivasta@google.com>
Change-Id: I9bdeaa59de5c58d11c0ae0e7b55d68d9e4d77fa1

# Conflicts:
#	src/daemon/metrics_list_test.c