REP-5329 Improve retryer & apply it more consistently.

[FGasper]

This changeset adds additional error codes to the retryer’s list of transient errors.

It also applies the retryer in several new places:

• metadata prep at the beginning of check
• generation of recheck tasks
• clearing of the recheck queue
• fetching failed & incomplete tasks
• find the next worker task
• get verification status
• inserting a partition verification task
• inserting a recheck task
• updating a verification task
• creating primary task
• setting the primary task to complete

A few functions are renamed for clarity as well, and some function calls are rewritten to read (hopefully) a bit more clearly.

[tdq45gj]

LGTM