Calling DB::open() with create_missing_column_families=true is quadratic

[sadderchris]

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Expected behavior

Per the notes here, when calling DB::open() with create_missing_column_families=true, column family creation is batched and takes a reasonable amount of time when opening a fresh database with lots of new column families.

Actual behavior

Calling DB::open() with create_missing_column_families=true doesn't actually batch create new column families and instead is quadratic in the number of column families it needs to create.

https://github.com/facebook/rocksdb/blob/8b566964b8b30965ed5e70115921705379b750fe/db/db_impl/db_impl_open.cc#L2056-L2060

Steps to reproduce the behavior

1. Set options.create_missing_column_families = true
2. Call DB::Open() on an empty directory and request to open a few hundred column families
3. Observe that the time needed to open is between 30s - several minutes