Summary: Added min/max/avg data block size output to sst_dump. Output was added to the end of BlockBasedTable::DumpDataBlocks, so it appears after the data block details, at the very end of the dump file.
Test Plan:
```
./db_bench --benchmarks=fillrandom
./sst_dump --file=/tmp/rocksdbtest-xyz/dbbench/000007.sst --command=raw
tail -n 6 /tmp/rocksdbtest-xyz/dbbench/000007_dump.txt
```
```
Data Block Summary:
--------------------------------------
# data blocks: 11336
min data block size: 903
max data block size: 2268
avg data block size: 2245.363356
```
Reviewers: IslamAbdelRahman
Reviewed By: IslamAbdelRahman
Subscribers: andrewkr, dhruba
Differential Revision: https://reviews.facebook.net/D61815
deda159b55