Fix pool active oracles and oracle health metrics lagging when pool is down

[greenhat]

Close #307
Close #308

Determine oracle health and active oracles count in the pool based on the last oracle's box height rather than pool box height.

[coveralls]

Pull Request Test Coverage Report for Build 5692625648

• 0 of 20 (0.0%) changed or added relevant lines in 3 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 54.448%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
core/src/metrics.rs	0	2	0.0%
core/src/api.rs	0	4	0.0%
core/src/monitor.rs	0	14	0.0%

Files with Coverage Reduction	New Missed Lines	%
core/src/api.rs	2	0%
core/src/monitor.rs	3	0%

Totals
Change from base Build 5643433554:	-0.2%
Covered Lines:	2093
Relevant Lines:	3844

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💛 - Coveralls