Add GATTC busy check?

I've triggered a bug (in my code) where I'm sending multiple GATT operations at once. I'm doing an MTU exchange request and a discover services request at once. Other BLE stacks I've used will return a BUSY error for the second request. Nimble doesn't have these checks. Is this by design or would a PR be accepted to add these? Thanks!

Start by reading the GATTC operation handling for MTU exchange and service discovery, then determine whether concurrent GATT operations should return BUSY. Done requires an agreed behavior and verification that a second operation receives the expected BUSY response; the issue names no files or tests.

Written by the indexing model from the issue text.