Transaction support

[Baughn]

With the supplied 'qdb' wrapper, if you do multiple simultaneous invocations of quinedb (as, say, to increase performance on a multicore system), there's a chance of clashing writes to qdb.out/err, which will break the executable entirely.

To get around this you should lock the output files before quinedb is invoked.

Also it would be nice to have it support transactions.

[gfredericks]

Would accept a pull request that does file locking. Maybe in a section at the bottom of the README for advanced users.

[NHOrus]

Suggest using sponge from moreutils to finalize transaction. As it automatically preserves executable bits.

[gfredericks]

I'd rather not add dependencies like that. Would make it a pain for installation.

[Baughn]

How about if we use nix-shell to automatically install sponge?

[gfredericks]

no.