Redesign statistics context and manager

Redesign statistics context and manager

- To simplify statistics context and statistics manager code,
have better control over statistics polling and to remove
referencing of statistics manager from statistics context,
remove current method of statistics scheduling and replace
it with Guava scheduled service that will handle starting,
periodic gathering and stopping of statistics.
- Simplify chain of steps that continues with statistics initialization
with reconciliation framework enabled

See also: bug 8913

Change-Id: I0f3dcdd5292c6c7180ea80afc6d4d3df16b05ff9
Signed-off-by: Tomas Slusny <tomas.slusny@pantheon.tech>