[1.x] Allow supplying Executor for SystemWriteSide

[yevhenii-nadtochii]

This PR addresses #1275.

Now, SystemSettings allows customizing an Executor which is used to post the system events in parallel. This provides an opportunity to improve the control over the available CPU resources on a server instance.

An example usage:

var builder = BoundedContextBuilder.assumingTests();
var executor = ...;

builder.systemSettings()
    .enableParallelPosting()
    .useCustomExecutor(executor);

The issue asked for ExecutorService that is extended version of Executor. But in the scope of SystemWriteSide and its single implementation DefaultSystemWriteSide, we can do with just Executor as no methods beside execute(Runnable) are used.

The library version is bumped to 1.8.2.-SNAPSHOT.1.

[yevhenii-nadtochii]

@armiol Please take a look.

[armiol]

LGTM with some minor comments to address.