Support microblock mining in neon nodes:
- On winning a sortition, a miner will attempt to assemble one microblock (if
mine_microblocksis set to true)
- Neon miners now will sleep a configurable amount of time before trying to mine a new anchor block (this gives a miner time to receive any microblocks that may be sent from the previous block's winner)
This also adds a new struct to miner.rs,
StacksMicroblockBuilder, since the microblock assembly has to outlive the StacksBlockBuilder instance. If we want to mine more than one microblock, a single instance of this struct needs to process each of those microblocks (since its ClarityTx isn't committed).
Additionally, this adds some integration testing structure for neon:
$ cd testnet $ BITCOIND_TEST=1 cargo test -- --test-threads 1 --ignored --nocapture neon_integrations
With an integration test for mining a microblock.