Do not repeatedly check fully-verifiable conditions in `crosshair watch`

[pschanely]

We shouldn't have watch repeatedly check postconditions that have been fully verified once - it's wasteful! (but we do need to ensure they are re-checked on a file change)

The messaging/stats should also report this, and needs to handle the rare case that every watched condition has been fully verified.

This came up in https://github.com/pschanely/CrossHair/issues/131#issuecomment-950392460_