[PATCH] Using mimalloc to improve performance and reduce memory allocation lock contention

We have a multi-threaded Windows application that was experiencing delays due to high lock contention in memory allocations from the PostgreSQL ODBC driver. We tried modifying the driver to use mimalloc, which is a memory allocator with better performance characteristics. After deploying this change, the delays due to lock contention disappeared. It has been running on thousands of our production deployments for 9 months without issue.

I've created this pull request so that others can benefit from this change by building the driver with the _MIMALLOC_ symbol defined and linking to the mimalloc library.