feat(make_idempotent): support making incr request idempotent in pegasus_write_service

[empiredan]

In #2185, we've implemented two APIs
make_idempotent() and put() in pegasus_write_service::impl to support idempotent
incr operations. As the higher-level abstraction, pegasus_write_service is able to process
idempotent incr requests. It will also implement make_idempotent() and put() by calling
APIs provided by pegasus_write_service::impl internally. Similarly, both APIs are actually also
provided for higher-level abstraction.

[acelyc111]

This is a member variable, which will be shared by all requests. But it's a request related latency, isn't it?

[empiredan]

This variable is defined as per-replica rather than per-request, for the reason that the current design for implementing idempotence is to make sure there is only one atomic request being processed in the write pipeline for each replica. This pipeline consists of the following stages:
(1) read the current value from RocksDB and built the idempotent request based on it;
(2) append the corresponding mutation to plog;
(3) broadcast the prepare requests;
(4) apply the result for atomic operation back to RocksDB ultimately.

For a request, this variable will be set in stage (1) and read in stage (4); since there is only one request in the pipeline, this variable is guaranteed not to be set for another request before stage (4) is finished. Therefore, it is safe to define this variable as per-replica.

[acelyc111]

The function name is not accurate, not only 'clean up', but also update the metrics.

[empiredan]

We could name this function batch_finish() as opposed to batch_prepare: finish batch write with metrics such as latencies calculated and some states cleared.