peer discovery by shard status with flexible url prefix

[brenzi]

closes #1524
closes #1529

Instead of broadcasting sidechain blocks to all sovereign enclaves of any MRENCLAVE, this PR changes the behavior to restrict broadcast only to enclaves who have touched the shard before.

We hereby introduce ShardConfig for this, which also allows custom shards to be initiated by any enclave.
In the future, this can be extended to have more control over the validateer set for a shard.

I also sneaked in other minor things:

• reattempting shard vault creation, a first step towards fail if primary validateer can't successfully create shard vault account #1530
• use new node version with 6s blocktime

[clangenb]

Looks good to me, but I have one question, regarding the approach in general.