Updating GATTServer's Connect/Disconnect calls
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Added the missing [Step 5.2.3](https://github.com/servo/servo/compare/master...szeged:connect-disconnect-update#diff-1dbe29f87740f5aec93f37adbecace6cR213) to the `connect` function.
Updated the [disconnect](https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetoothremotegattserver-disconnect) function to its current state in the specification, including the `clean_up_disconnected_device` and the `garbage_collect_the connection` functions.
Added new tests for checking the invalid state of JS objects after disconnection.
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- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes
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They're not at all related, besides both being asynchronous. This change adds
a little extra code in response_async(), but makes this code more readable
and reduces the unnecessary indirection.
This change also makes the build system slightly more parallel, by dropping
the dependency on bluetooth_traits from net_traits.
It could be used to have mutable JSVal fields without GC barriers.
With the removal of that trait, MutHeap and MutNullableHeap can respectively
be replaced by MutJS and MutNullableJS.
