Add Benchmarks to compare with native modules

[ethanfrey]

BankMsg and ExecuteMsg on cw20-base is the simplest comparison.

Starting with the benchmark in x/bank, let's:

• batch multiple messages per block (eg. 20) (Initial benchmarks of wasm contracts compared to sdk modules #638)
• use leveldb as a backing store (More realistic benchmarks #639)
• set up the equivalent for cw20 (use the same setup, just different messages) (Initial benchmarks of wasm contracts compared to sdk modules #638)
• set up significant state first (eg. 10k accounts for both bank and cw20) (More realistic benchmarks #639)
• have bank messages send from different pairs of accounts (not all same sender and same receiver) (More realistic benchmarks #639)
• Run in go-bencher

[ethanfrey]

Basically done.

One last follow up in #652