Revert "Back numberOfSendingWorkerThread and related method into Config"

This reverts commit 23df5f1e7d1f9fc0d77c0594a78c2b77a36f0327.
Set transient to deprecated field
Remove interface class for PluginConfig
Fix checkstyle error
Add setter method

To make changes minimize, setter method is added to interface class.
This should be removed in future. Also added to Config class.
Copy setting in PluginConfig to Config

NumberOfSendingWorkerThreads is moved to PluginConfig. But ones in Config
is still needed for several process. Unfortunately it is difficult to
replace easily.

So copy value from PluginConfig to Config whenever Config instance is
created and PluginConfig instance is updated.

Note that this is workaround. We should adopt another solution in future.
Move shutdown process for GerritSendCommandQueue

It is inefficient that GerritSendCommandQueue is shutdown whenever one
of server is shutdown.

This patch moves shutdown process to PluginImpl.stop().
Implement additional interface method to mock