Properly await throughput recording in MonitoringService

[johnsimons]

Problem

MonitoringService.RecordMonitoringThroughput iterated the endpoints in an incoming
throughput message with List<T>.ForEach:

message.EndpointThroughputData?.ToList().ForEach(async e => { ... await ... });

ForEach takes an Action<T>, so the lambda was async void. That has three consequences:

1. RecordMonitoringThroughput returned as soon as the first await suspended, so the
   transport handler acknowledged the message while the writes were still outstanding. If
   the process stopped in between, the data was gone and the message was not redelivered.
2. Exceptions inside the lambda were unobserved instead of surfacing to the handler.
3. Every endpoint in a message was written concurrently against the same data store.

This is the path where a Monitoring instance reports throughput to the primary instance, and
that data feeds the usage report customers send to Particular. Losing it means under reported
usage.

How it was found

This came out of the EF acceptance tests work. It was not caught by the existing unit
tests because each of them sends a message containing a single endpoint, so there is nothing
to lose.