Make project export to be of constant memory

Building the hash of project export in memory is very expensive operation.

We can significantly improve it by serializing and writing to JSON at the same time instead of storing Hash representation in memory.

This allows us to quickly allocate and quickly free memory used by export, instead of retaining it for longer period of time.

Original description

It seems that the biggest cost of export is keeping in-memory structure of hash. When I tested the hash size I noticed this to take 250MB for gitlabhq. This is first step, then during save, we effectively duplicate and rewrite that hash. Then we serialize this hash to string. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32423/diffs?commit_id=a33994318c3212a49338553e83bc4709271648a6 > this makes to be serializer almost of constant size, except the memory growth related to storing string with json. We serialize small chunks, and forget the hash.

I was able to get down from 1.2G after export to 510MB. The memory allocated did hover around 600MB during the export.

But so far it seems that we can:

1. Shorten export by about 4x times with N+1 fix,
2. We can reduce memory footprint (for gitlabhq) from around 700MB (on top of process), to around 150MB.

Related to: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/32579

Confirmed by @alipniagov https://gitlab.com/gitlab-org/gitlab-ce/issues/35389#note_211235853.