Benchmarks crash on goleveldb about iavl

[ethanfrey]

I can reproduce this also on develop: commit 595f3dc

cd benchmarks/ && go test -bench=Medium

[ethanfrey]

Seems like some crashed tests left half-finished dbs there. With help from @cloudhead I removed them and now it works...

$ go test -bench=Medium
Init Tree took 96.26 MB
goos: linux
goarch: amd64
pkg: github.com/tendermint/iavl/benchmarks
BenchmarkMedium/memdb-100000-100-16-40/query-miss-8         	  200000	      5265 ns/op
BenchmarkMedium/memdb-100000-100-16-40/query-hits-8         	  300000	      6180 ns/op
BenchmarkMedium/memdb-100000-100-16-40/update-8             	   10000	    283194 ns/op
BenchmarkMedium/memdb-100000-100-16-40/block-8              	      50	  29689677 ns/op
Init Tree took 46.96 MB
BenchmarkMedium/goleveldb-100000-100-16-40/query-miss-8     	  100000	     12652 ns/op
BenchmarkMedium/goleveldb-100000-100-16-40/query-hits-8     	  100000	     16212 ns/op
BenchmarkMedium/goleveldb-100000-100-16-40/update-8         	   10000	    330389 ns/op
BenchmarkMedium/goleveldb-100000-100-16-40/block-8          	      50	  38318145 ns/op
Init Tree took 38.89 MB
BenchmarkMedium/leveldb-100000-100-16-40/query-miss-8       	  100000	     12147 ns/op
BenchmarkMedium/leveldb-100000-100-16-40/query-hits-8       	  100000	     15993 ns/op
BenchmarkMedium/leveldb-100000-100-16-40/update-8           	   10000	    336612 ns/op
BenchmarkMedium/leveldb-100000-100-16-40/block-8            	      30	  33432249 ns/op
PASS
ok  	github.com/tendermint/iavl/benchmarks	32.400s