Previously SearchMetricNames was returning unmarshaled metric names.
This wasn't great for vmstorage, which should spend additional CPU time
for marshaling the metric names before sending them to vmselect.
While at it, remove possible duplicate metric names, which could occur when
multiple samples for new time series are ingested via concurrent requests.
Also sort the metric names before returning them to the client.
This simplifies debugging of the returned metric names across repeated requests to /api/v1/series
This opens doors for implementing vmselect api server at vmselect level,
so top-level vmselect could query lower-level vmselect nodes in the same way
as it queries vmstorage nodes.
This will create the ability to create highly available querying architecture
when multiple independent VictoriaMetrics clusters with the same data
are located in distinct availability zones. In this case we can use top-level
vmselect instead of Promxy for simultaneous querying of all the clusters
in all the AZs.
