Update bulk_update of GemDownload to use single query for versions find
Update bulk_update of GemDownload to use single query for versions find

Stats job SLO alert has been triggering quite frequently, specially
during peak traffic hours because of higher log volume.

Some files have upwards of 5000 unique versions and fetching versions
for them in separate queries can be quite slow. Fetching all versions
of the log file makes FastlyLogProcessor.perform almost twice as faster:
```
== before
$ time rails runner 'FastlyLogProcessor.new("some", "thing").perform'

real    0m20.198s
user    0m15.214s
sys     0m1.020s

== after
$ time rails runner 'FastlyLogProcessor.new("some", "thing").perform'

real    0m11.565s
user    0m8.212s
sys     0m0.581s
```

log file was read from local.

rubygems/rubygems.org,sonalkr132/rubygems.org,sonalkr132/rubygems.org,rubygems/rubygems.org,rubygems/rubygems.org,rubygems/rubygems.org,sonalkr132/rubygems.org,sonalkr132/rubygems.org