Make library thread safe (SQL providers)

[videokojot]

Tracking issue:
https://github.com/borisdj/EFCore.BulkExtensions/issues/1284

During my work on MIT fork of this repository https://github.com/videokojot/EFCore.BulkExtensions.MIT I discovered that the library is not thread safe. The SQL adapter is saved in static field, so in case of two threads using the library against two different SQL providers at the same time, this would lead race condition and unexpected errors.

I fixed this in: videokojot/EFCore.BulkExtensions.MIT#22 by getting the provider from the DbContext used for operation (which is available) and for performance we cache the providers in the static fields (but these do not hold any DbContext instance specific data).

[videokojot]

#22