add database-to-database push/pull

For a first pass, this operator can be manually-invoked. ":push " and ":pull " could be implemented which merge in remote transactions from the remote branch of the same name.

For a pull, first, a new branch of "remote/" is created and the remote transactions downloaded, then a UnionPreferMerge is performed.

For a push, the same steps are performed on the remote server.

Once this feature is implemented, it will be possible to automate the pushes and pulls to simulate multi-master replication. Multi-master replication will need either: 1) merge strategies which can change the transaction graph (removing transactions?) 2) runtime Haskell merge strategies.