Use NativeLibrary.TryLoad and prefer RTLD_LAZY over RTLD_NOW (dotnet/corefx#35548)

Use NativeLibrary.TryLoad and prefer RTLD_LAZY over RTLD_NOW (dotnet/corefx#35548)

* Use NativeLibrary.TryLoad in ODBC test

* Use RTLD_LAZY instead of RTLD_NOW

* Add missing using statement

* More RTLD_NOW -> RTLD_LAZY

* Remove unused RTLD_NOW constant

* Use NativeLibrary.TryLoad instead of P/Invoke dlopen

* Inline method body in property

* Add fallback for netcoreapp2x and below

* Reduce visibility of const

Commit migrated from https://github.com/dotnet/corefx/commit/863b9bff2d77669b426b52dcb1bf7ba9246729e0