Subnet miner should wait before first commit

**Describe the bug**

When using `clarinet integrate` to spin up a subnet network, the subnet miner submits block commitments _before_ its reached the chain tip of the L1 stacks network. This leads to the first commitment failing with `(ERR 11)` (invalid chain tip).

There's a bunch of different ways this could be tackled -- here's the two that occur to me initially:

1. The subnet node could use the L1 RPC interface to get the current block height, and wait until its processed up to that height.
2. The subnet node could use the "wait until height" configuration parameter.