Implement Send for structs that wrap GDAL types

[ttencate]

Quoting the documentation:

a C function or C++ method is said to be re-entrant if it can be called simultaneously from multiple threads, but only if each invocation uses its own data.

All GDAL public C functions and C++ methods are re-entrant, except:

• the general initialization functions, like GDALAllRegister().
• the general cleanup functions like GDALDestroy() or OSRCleanup().

Thus, it should be safe to implement Send for all wrapper types. This was already done for Dataset in #99, OwnedLayer in #238 and Error in #293.

The one I'm currently missing is SpatialRef, but there are probably more.

[BlakeOrth]

Given the discussion in #419 and the discovery of the underlying behavior of some of the reference counted types, it seems like careful consideration needs to be made when marking GDAL struct wrappers as Send. As my #419 (comment) suggests, I've done a decent amount of digging for SpatialRef specifically and I believe with some (API breaking) changes it can safely be marked as Send. I'd be happy to open a PR with those changes if you'd be interested in introducing them.