hydra-notify has a very slow startup

[srhb]

Even when doing nothing (eg. supplying an unknown or no command) startup can take more than a second. Due to this, hydra.nixos.org can be hundreds of thousands of notifications behind.

Perhaps batching notifications could be beneficial, but this might complicate the queue.

[edolstra]

We should make hydra-notify a persistent child process of the queue runner that receives notification items from stdin, or something like that.