feat: callback to intercept and modify predicates before re-registration

[zone117x]

Closes #674

• First commit adds an optional callback that clients can use to modify an inactive predicate before it is re-registered.
• Second commit fixes an issue where the the setInterval could result in overlapping healthcheck code running if something like the callback (which could be waiting on a postgres connection, for example) take a long time.

[rafaelcr]

LGTM