subserver+status: check map entries before adding new servers #877
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For the subserver manager: use a map instead of an array for subservers. Check that there is no name collision before adding a new server. This is also nice if we want to get a specific subserver by its name (as is done in Prepare for custom channels (lnd v0.18.4-beta) #848).
for the status manager, first check that we are not overriding an existing subserver before we register a new one