Removed Cache from metrics flow for Block::MetricsIter to DBImpl

Removed Cache from metrics flow for Block::MetricsIter to DBImpl

Summary:
Before this patch we had to send metrics to `Cache`, which then sent the metrics to `DBImpl`. Now we directly send them to `DBImpl`, by the use of a pointer to a pointer of a `BlockMetrics` instance. We can do this safely and without additional locks because:

* `Block::MetricsIter` runs in the same thread as `DBImpl::Get()` (in which the `BlockMetrics` pointer resides)
* `DBImpl::Get()` locks `DBImpl` again after the `Block::MetricsIter` is destroyed, so we don't have to do an additional lock.

Test Plan: make check

Reviewers: vamsi, dhruba

Reviewed By: vamsi

CC: leveldb
Differential Revision: https://reviews.facebook.net/D8583