measure deferred_decl bucket sizes

measure deferred_decl bucket sizes

Summary:
The earlier diff in this stack introduced telemetry about job sizes from the multiworker typechecker.

I noticed a discrepancy there, when I did a full init, where it claimed the total number of files processed was larger than the number of files in WWW.

The reason for that discrepancy is deferred_decls. It did indeed "process" more files than there are files in WWW, but it only *completed typechecking* of the same number of files as in WWW.

This diff adds telemetry about deferred decls:
* how many files all-up were processed
* how many Check workitems were completed, and how many were deferred until later, and for that deferral how many Decl items were queued up for later
* how many Prefetch workitems were done
* how many Decl workitems were done

The numbers all look reasonable, and pretty much what you'd expect. The total number of checks_done adds up to the total number of php files in WWW.
```
1596 finished_to_end.checks_done.count * 876.543 finished_to_end.checks_done.mean = 1,398,963 finished_to_end.checks_done
2652 finished_early.checks_done.count * 456.7 finished_early.checks_done.mean = 1,211,251 finished_early.checks_done

SUM = 2,422,051 files
```

Reviewed By: CatherineGasnier

Differential Revision: D24771690

fbshipit-source-id: f296e0a75d82543fe9380103a1b5afdd53067a36