ACID Transactions

[dhardy]

This was not one of the original design criteria, but should be achievable. It already is in a way within a single partition, by selecting "A" or "B" to reject one head when merging (but which head should be rejected may not be obvious).

• Single-partition transactions: try-add-and-merge-commit
• Multi-partition transactions (1): lock all [affected] partitions
• Multi-partition transactions (2): test whether a commit has conflicts
• Multi-partition transactions (3): test each merge is possible, then merge or reject