Implement Multithreading Support

When I create a single environment handle in the main thread and then spawn several threads, each with its own connection, and execute queries, the driver often crashes. Note that each thread uses its connection exclusively, i.e., connections are not shared between threads. I do not use any ODBC wrappers, only a C++ application using the native ODBC API.
Here are errors I frequently receive on Linux using unixODBC:

- `Error: HY010: [unixODBC][Driver Manager]Function sequence error`
- `Segmentation fault`
- `double free or corruption (!prev)`
- The application gets stuck

Errors I receive on Windows:

- The application often gets stuck
- The application crashes, but in the Event Log I can see an entry with error `Application Error 0xc0000005 (Access Violation)`
- `Error: 08003: [Microsoft][ODBC Driver Manager] Connection not open`

Exactly the same example works perfectly well with both PostgreSQL and MySQL drivers on the same data.