Support instant seal

[JoshOrndorff]

This PR adds instant seal and interval seal as authoring schemes in the --dev service. This will allow users to easily run an instant seal node for testing their local development against moonbeam, while also allowing testing of block-time based things like staking and democracy periods, and also supporting manual seal for integration tests.

# Manual seal like we use in the tests, and when reproducing complex forking behavior
moonbase-alphanet --dev --tmp --sealing manual

# Instant seal for testing your contracts, bots, dapp, etc really fast like ganache
moonbease-alphanet --dev --tmp --sealing instant

# Interval seal for when you want blocks to come at a fixed time interval
# The parameter is miliseconds. So this will author every 6 seconds
# Useful for testing stuff that happens in block time eg. democracy
moonbase-alphanet --dev --tmp --sealing 6000

# The default is instant seal
moonbase-alphanet --dev --tmp

Checklist

• ❌ Does it require a purge of the network?
• ❌ You bumped the runtime version if there are breaking changes in the runtime ?
• ✔️ Does it require changes in documentation/tutorials ?

[crystalin]

Awesome :)