Add interruptblock RPC

[Jouzo]

Summary

• Introduces interruptblock RPC, which takes a block height as only param.
• If a node is running without -interrupt-block startup flag, sets fInterrupt and fInterruptBlockHeight.
• Can be run multiple times in a row and will update fInterruptBlockHeight to the latest height passed to interruptblock RPC.

This let's a user easily run a command against a target block height.
Example usage :

for height in `5000 10000 15000`
   defi-cli interruptblock height
   wait_for_block height
   ... Run any command at  required height.

Idea behind it is to ease diff log generation for full sync CI or Ocean A/B testing.
Current node behaviour requires multiple restart (as interrupt-block height can only be set via startup flag).
This also lets one interrupt an already running node.

[prasannavl]

This region is un-synchronized. Would put this under the lock too.

    fInterruptBlockHeight = height;
    fInterrupt = true;

[prasannavl]

Actually most of this needs the lock until the last activate step.

I'd actually just simplify it and take the lock as a whole than attempting to be granular here.