Warn about THP after startup

If THP is enabled, MaxScale should log a warning stating that it might use more memory than is expected.

For a future release, once static buffers have been allocated, all runtime buffers could disable use of transparent hugepages. The query classifier cache is expected to take up notable amounts of memory and it should not use THP as it is not optimized for it. Some of the buffers in the query canonicalization code could benefit from it but the benefits of it ought to be first measured.

The effects of THP should also be evaluated to estimate if there is any performance benefit to enabling it in general.