Function to check if an ID is occupied by any channel inside the DB.

[adi2011]

Some nodes complained about UNIQUE constraint failed upon entering recovery mode. Which means they had closed channels inside the DB which were not loaded hence emergencyrecover tried to stub channels at the same ID. So we need to check for occupied channels inside the DB.

It solves: #7187 #7162 #7183

[vincenzopalazzo]

ACK 0d5ba47

CI failure unrelated

===End Flaky Test Report===
=========================== short test summary info ============================
ERROR tests/test_renepay.py::test_limits - ValueError: 
Node errors:
 - lightningd-1: had memleak messages
Global errors:

[endothermicdev]

I see this one under valgrind. This was on reconnect, so looks suspicious.

FAILED tests/test_misc.py::test_recover_plugin - pyln.client.lightning.RpcError: RPC call failed: method: connect, payload: {'id': '0266e4598d1d3c415f572a8488830b60f7e744ed9235eb0b1ba93283b315c03518', 'host': 'localhost', 'port': 42621}, error: {'code': 402, 'message': 'disconnected during connection'}

[vincenzopalazzo]

To me the error disconnected during connection is like a bug in the test code (?) but still unrelated to this PR I think.

This can happen when you call disconnect after a connect, but I need to look at the test code before judge, sorry for this early and uncheck feedback

[endothermicdev]

Rebased now that the renepay test is fixed.

[endothermicdev]

ACK f8680c0