[Introspection] Investigate and improve introspection speed

[janpio]

Right now Introspection sometimes fails for big schemas (See #1493). We should investigate and improve the behavior of Introspection by e.g. parallelizing queries or similar.

[tomhoule]

Magento has ~400 tables, we're doing ~1200 SQL queries to introspect at the moment (see the following log).

logs.txt

We should be able to cut the number of queries by quite a lot with a small amount of work, so I'll try that.

[tomhoule]

After a first series of PRs, the number of introspection queries on the same database is down to 5 (from ~1400).

[tomhoule]

Current status:

• we optimized introspection queries on MySQL and now it is fast™
• we optimized one query on postgres (columns), but the others are more complicated, so we will do it when we have time or it becomes a necessity