Add an option database_type

[n-rodriguez]

Problem: When using numeric_ordering: true, closure_tree tries to find out the type of database during the initialization process. This can be problematic, for example when doing rails db:create, in which case there is no database, yet. Another scenario is rails asssets:prcompile in a Docker container that is being made for production, but it doesn't have access to the production database yet, because it isn't rolled out, yet.

Solution: I have added an option to specify the database_type manually. This option is only used then numeric_ordering is set to true

[n-rodriguez]

This PR is a rebase on master branch of #338.

It closes #344 and #338

[n-rodriguez]

On my side everything is green : https://github.com/n-rodriguez/closure_tree/actions/runs/491731739

ping @seuros

[n-rodriguez]

well.... there might be an other way of doing it by using load_schema!

I'll see what I can do

[timdown]

Is there any chance this could be used to add in database-specific queries? For example, a MySQL-specific query for faster hierarchy deletion (see #343)

[kbrock]

Is there any chance this could be used to add in database-specific queries? For example, a MySQL-specific query for faster hierarchy deletion (see #343)
-- @timdown

FWIW/ ancestry uses this concept to optimize database specific optimizations.
think this is a great start down that road.