Multi node pipeline executor

[dolavb]

This allow clients to provide an ExecutorService at pipeline creation instead of having a new ExecutorService created at every pipeline Sync call. An Executor service creation will create new threads, which are expensive resource to creates. In a high throughput application developed internally, we are writing at a rate of ~100k set per seconds, on a six node cluster, on an instance equivalent to an EC2 m5.12xlarge. The creation of threads uses 40% of CPU, and adds substantial latency. This new approach will allow clients to send a pooled Executor that is tuned to there load patterns.

This change is non breaking, but will come with a slight optimization for the clients currently using the created thread pool. In the current approach even if a pipeline has a single connection to close the Executor service will create MULTI_NODE_PIPELINE_SYNC_WORKERS threads. In the default mode would mean wasting 2 thread creation.

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From: https://stackoverflow.com/questions/5483047/why-is-creating-a-thread-said-to-be-expensive

Thread lifecycle overhead. Thread creation and teardown are not free. The actual overhead varies across platforms, but thread creation takes time, introducing latency into request processing, and requires some processing activity by the JVM and OS. If requests are frequent and lightweight, as in most server applications, creating a new thread for each request can consume significant computing resources.

From Java Concurrency in Practice
By Brian Goetz, Tim Peierls, Joshua Bloch, Joseph Bowbeer, David Holmes, Doug Lea
Print ISBN-10: 0-321-34960-1

Java thread creation is expensive because there is a fair bit of work involved:

• A large block of memory has to be allocated and initialized for the thread stack.
• System calls need to be made to create / register the native thread with the host OS.
• Descriptors need to be created, initialized and added to JVM-internal data structures.

[sazzad16]

@dolavb Thank you for your effort to improve Jedis.
Your concern is justified. But we have our hands full ATM. We'll try to get to this PR ASAP.