Use WriteBatch for Put operation in badger DB.

[martinmr]

This reduces the running time by a lot. In my machine (with 1M keys and
two go routines) the Put running time went from ~20s to ~4s.

[recoilme]

Hello, @martinmr !
But other databases may batch writing operations too. I think use batching only for badger - will not fair..
May you please:

• add batching for goleveldb and bbolt?
• implement batching as an command line option for the test?

[martinmr]

I have added batching for goleveldb and bbolt.

With respect to the flag, I don't think it's needed. If batching is available, it should be the only default. At least with badger, using a transaction per put is not the way serial transactions would be made in a real-use case so using that procedure for benchmarks would result in inaccurate results. I assume it's the same for other dbs.

[recoilme]

Thank you for adding batching, looks good!
I merge it now but test a little later. But i think code like this:

start batch
insert millions key/value
write batch (write insertions on disk, with fsync())

Far from the real-world use case. Because without fsync all writes may be lost (they are stored in disk-cache before fsync, but i think you know it). On the other hand, batching very useful when you write some logs or data by portions or import database from a backup for example.

So, i think flag for batching is needed