Move benchmarks to Level/bench

[vweevers]

As a new home for both abstract-leveldown and levelup benchmarks.

[vweevers]

Added ioredis and sqlite3 support: https://github.com/Level/bench/tree/master/third-party

[vweevers]

Added a batch-put benchmark: Level/bench#4

[vweevers]

Remaining:

• get benchmark: Add get() benchmark bench#15
• iterator benchmark: Add get() benchmark bench#15
• readStream benchmark