Implement Irmin.compare_and_set

[samoht]

This is actually currently only implemented for the RW backend as it is the basis to ensure that Irmin.update is consistent. Could be useful to implement it in the high-level API though (instead of throwing a "TODO") though.

[samoht]

S.with_tree, S.set, S.set_treeandS.removenow have an optional argument?strategy` which can either be:

• Set: "last writer wins" which was the only option in pre-1.0.
• Test_and_set: atomic update of the subtree, checking if the previous subtree has changed and retrying the transaction if that's the case.