Tenants get their own connections for statement verbs and are further
isolated from each other by different scheduling groups. A tenant is
identified by a scheduling group and a name. When selecting the client
index for a statement verb, we look up the tenant whose scheduling group
matches the current one. This scheduling group is persisted across the
RPC call, using the name to identify the tenant on the remote end, where
a reverse lookup (name -> scheduling group) happens.
Instead of a single scheduling group to be used for all statement verbs,
messaging_service::scheduling_config now contains a list of tenants. The
first among these is the default tenant, the one we use when the current
scheduling group doesn't match that of any configured tenant.
To make this mapping easier, we reshuffle the client index assignment,
such that statement and statement-ack verbs have the idx 2 and 3
respectively, instead of 0 and 3.
The tenant configuration is configured at message service construction
time and cannot be changed after. Adding such capability should be easy
but is not needed for query classification, the current user of the
tenant concept.
Currently two tenants are configured: $user (default tenant) and
$system.
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