Calling margo_provider_register multiple times with different pools

[mdorier]

Mercury allows calling HG_Register multiple times on the same hg_id_t if we want to change the callback and/or the serialization functions. Looking at the margo code, I'm under the impression that if we do that via margo_provider_register and change the Argobots pool associated with the RPC, the pool won't be changed. See here: https://github.com/mochi-hpc/mochi-margo/blob/main/src/margo-core.c#L1468

[carns]

The fix for this would be easy (just reset the fields in the existing margo_data if found). The harder part will be to create a test case :) Is there a specific example we want to verify works?