Rework exitwhensynced functionality

[uprendis]

Add --exitwhensynced.age and --exitwhensynced.epoch flags instead of --exitwhensynced flag:

• --exitwhensynced.age may be used instead of --exitwhensynced to stop node once it syncs up to a relatively latest state. --exitwhensynced.age requires to specify an exact age threshold of what's considered a latest state (e.g. 5s)
• --exitwhensynced.epoch allows to stop node exactly when it reaches a certain epoch. Its intended usecase is genesis files generation for a certain epoch

[hadv]

it sound a good feature, May we add more --exitwhensynced.block to stop node at a specific block height. it might be the use-cases for end-user due to account state is updated in block by block basis.

[uprendis]

Unfortunately stopping node a specific block height wouldn't be trivial to implement. The atomic consensus message is event. Processing single event may lead to processing multiple blocks, so it's clear how to stop at some height >= condition but not exactly ==