These signals are usually named `interruptor`, per @srh.
Initialize these before visitor stuff.
Adjust comment to reflect that this is used for network serialization too.
Per @srh, unlimited compilation sizes.
Tighten whitespace to make for closer symmetry.
Per @srh, break symmetry here and only ship around limits.
Satisfy demands to write this to disk.

batched_insert_t wants to write to disk, and so we need to support
reading and writing to the old format.