storage/engine: change min_write_buffer_number_to_merge from 2 to 1
storage/engine: change min_write_buffer_number_to_merge from 2 to 1

Git history doesn't provide adequate justification for setting this
value to 2. This option controls how many "write buffers" before writing
to L0. The setting of 2 was sort of strange given that our write buffer
size is larger than our target L0 file size. So we would wait until we
had 2 write buffers (each 8 MB) and then create ~4 L0 files.

A higher value could be beneficial if there are duplicate records in
each of the individual write buffers. block_writer testing doesn't show
any perf benefit to the higher value.

veteranlu/cockroach,vaibhavarora/cockroach,petermattis/cockroach,petermattis/cockroach,cockroachdb/cockroach,maxlang/cockroach,veteranlu/cockroach,veteranlu/cockroach,cockroachdb/cockroach,paperstreet/cockroach,paperstreet/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,petermattis/cockroach,glycerine/cockroach,petermattis/cockroach,maxlang/cockroach,paperstreet/cockroach,glycerine/cockroach,veteranlu/cockroach,glycerine/cockroach,veteranlu/cockroach,maxlang/cockroach,maxlang/cockroach,paperstreet/cockroach,glycerine/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,glycerine/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,cockroachdb/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,glycerine/cockroach,petermattis/cockroach,petermattis/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,maxlang/cockroach,petermattis/cockroach,vaibhavarora/cockroach,vaibhavarora/cockroach,veteranlu/cockroach,maxlang/cockroach,petermattis/cockroach,maxlang/cockroach,cockroachdb/cockroach,paperstreet/cockroach,glycerine/cockroach,cockroachdb/cockroach,glycerine/cockroach,cockroachdb/cockroach