cluster: guard against undefined message handlers

[cjihrig]

Checklist

• tests and code linting passes
• the commit message follows commit guidelines

Affected core subsystem(s)

cluster

Description of change

cluster's internal message handling includes a cache of callback functions. Once the message for that callback is received, it is removed from the cache. If, for any reason, the same message ID is processed twice, the callback will be missing from the cache and cluster will try to call undefined as a function. This commit guards against this scenario.

Fixes #6561