Add an option which allows top stop/start Processing Thread Online

[kamilchodola]

This would be beneficial for testing.

Add some custom json rpc or other way of being able to stop processing thread on online node, and resume it at some point.

Scenario we want ot be able to replicate is:

1. Synced node
2. Stop Processing while CL is online and EL is online but responding with "Syncing"
3. Stack 1000 blocks in Processing queue
4. Resume the processing to process all stacked blocks.

[kamilchodola]

cc @asdacap @stdevMac

it may help on reproduction of op-stack trie exceptions