Allow cache_bench/db_bench to use a custom secondary cache

[anand1976]

This PR adds a -secondary_cache_uri option to the cache_bench and db_bench tools to allow the user to specify a custom secondary cache URI. The object registry is used to create an instance of the SecondaryCache object of the type specified in the URI.

The main cache_bench code is packaged into a separate library, similar to db_bench.

An example invocation of db_bench with a secondary cache URI -
db_bench --env_uri=ws://ws.flash_sandbox.vll1_2/ -db=anand/nvm_cache_2 -use_existing_db=true -benchmarks=readrandom -num=30000000 -key_size=32 -value_size=256 -use_direct_reads=true -cache_size=67108864 -cache_index_and_filter_blocks=true -secondary_cache_uri='cachelibwrapper://filename=/home/anand76/nvm_cache/cache_file;size=2147483648;regionSize=16777216;admPolicy=random;admProbability=1.0;volatileSize=8388608;bktPower=20;lockPower=12' -partition_index_and_filters=true -duration=1800

[pdillinger]

LGTM

Can you show an example command line in the commit/PR description?

Unfortunately we lose our git blame on cache_bench source code, but avoiding that with our PR infrastructure is a big pain.

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[facebook-github-bot]

@anand1976 merged this pull request in 13232e1.