bogn: add statistics.

[prataprc]

Many statistics information can be added into bogn for debugging purpose.

a. cumulative time taken to build disk snapshots.
b. total bytes of data written to disk.
c. total bytes of data ingested (including overhead) by memory level (mw).
d. cumulative time taken to ingest data by memory level (mw)

b/c shall give write amplification factor.
c/a shall give write ingestion rate.

PS: use this task to track all stats related commits.