the current transaction id is when you change it with this
compare-and-swap message.
+ reserve_metadata_snap
+
+ Reserve a copy of the data mapping btree for use by userland.
+ This allows userland to inspect the mappings as they were when
+ this message was executed. Use the pool's status command to
+ get the root block associated with the metadata snapshot.
+
+ release_metadata_snap
+
+ Release a previously reserved copy of the data mapping btree.
+
'thin' target
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