Resolve "The `populate_prod_db` is failing, leading to missing data in the Insights dashboards"

This improves the populate_prod_db job by:

• Fetching the API asynchronously in batch of 5 pages thanks to Concurrent::Future (before we'd fetch synchronously the API, waiting for each page to return before fetching the next one).
• Inserting the records as we retrieve them, minimizing the memory amount used bu the job (before we'd wait to have all the resources from the API before inserting them).

Problems remaining to solve

The memory used is constantly increasing and reaching 1.7GB when processing gitlab-foss, as can be seen here: https://gitlab.com/gitlab-org/gitlab-insights/-/jobs/344683057

              total        used        free      shared  buff/cache   available
Mem:           7983        7573         126         221         282          24
Swap:             0           0           0
memsize_of_all: 1794.234375 MB, ObjectSpace.count_objects: {:TOTAL=>22342887, :FREE=>5247592, :T_OBJECT=>3124075, :T_CLASS=>99398, :T_MODULE=>1039, :T_FLOAT=>9, :T_STRING=>8967399, :T_REGEXP=>1946, :T_ARRAY=>2423839, :T_HASH=>1971465, :T_STRUCT=>7234, :T_BIGNUM=>9, :T_FILE=>14, :T_DATA=>275813, :T_MATCH=>11024, :T_COMPLEX=>1, :T_RATIONAL=>27611, :T_SYMBOL=>514, :T_IMEMO=>182002, :T_NODE=>214, :T_ICLASS=>1689}

Data point checks

Project	Current	Review app	OK?
gitlab-org/projects/build/CNG	https://quality-dashboard.gitlap.com/groups/gitlab-org/projects/build%2FCNG/monthly_merged_mrs	https://151-improve-populate-prod-db-review.quality-dashboard.gitlap.com/groups/gitlab-org/projects/build%2FCNG/monthly_merged_mrs	✅
gitlab-org/charts/gitlab	https://quality-dashboard.gitlap.com/groups/gitlab-org/projects/charts%2Fgitlab/monthly_merged_mrs	https://151-improve-populate-prod-db-review.quality-dashboard.gitlap.com/groups/gitlab-org/projects/charts%2Fgitlab/monthly_merged_mrs	✅ (current data are missing)
gitlab-org/gitaly	https://quality-dashboard.gitlap.com/groups/gitlab-org/projects/gitaly/monthly_merged_mrs	https://151-improve-populate-prod-db-review.quality-dashboard.gitlap.com/groups/gitlab-org/projects/gitaly/monthly_merged_mrs	✅
gitlab-org/gitlab	https://quality-dashboard.gitlap.com/groups/gitlab-org/projects/gitlab/monthly_merged_mrs	https://151-improve-populate-prod-db-review.quality-dashboard.gitlap.com/groups/gitlab-org/projects/gitlab/monthly_merged_mrs	✅ (current data are missing)

Closes #151 (closed).