Eventing : Handle checkpointing failure by cursor_aware functions gracefully

Problem:

For cursor_aware functions, on a checkpointing failure other than CAS_MISMATCH, the function handler skips onto the next mutation silently.
A user has no mechanism:

• Of knowing whether a failure is ongoing and mutations are being skipped due to checkpointing failure
• Of knowing the document IDs of mutations for which checkpointing failed.
• To control the maximum time up to which the checkpointing is allowed to run.

Solutions:

• Expose dcp_mutation_checkpoint_failure and dcp_deletion_checkpoint_failure via prometheus endpoints so that they can be reported as failures in eventing stats and in the on-prem UI by ns_server.

• Push document IDs of failed mutations to app log.

• Adding a checkpoint_timeout (just like script timeout) to control time limit on checkpointing activity.