MessagePack structure bug in some queries

* System info: Linux 4.9.0-11-amd64 x86_64
* OS: Debian GNU/Linux 9.13 (stretch)
* InfluxDB version: 1.8.5 and 1.7.3

If you look at the issue I analyzed in [influxdb-python](https://github.com/influxdata/influxdb-python/issues/820#issuecomment-826893793_), you will see that there is a suspicion of a msgpack issue in the server.
It was uncovered when querying 'SHOW DIAGNOSTICS' but I also got the exact same problem in "some" queries (that would be a bit trickier to debug as debugging involved reading the binary msgpack structure and validating it by hand)

I'm unfortunately not that comfortable with go and couldn't really track down the problem in the server code...

Start with the linked influxdb-python issue and its analysis of the SHOW DIAGNOSTICS response, then reproduce the malformed MessagePack structure against the listed InfluxDB versions. Trace the server's query response serialization path and compare it with the manually validated binary structure. Done means the affected queries consistently produce valid, correctly structured MessagePack responses.

Written by the indexing model from the issue text.