Add possibility to set max-select-points on a session level

__Proposal:__
Add possibility to set max-select-points on a session-level, (like suggested [here](https://github.com/influxdata/influxdb/issues/5323) at the end) but without need to restart DB.

__Current behavior:__
Currently, you can set it in config by changing `max-select-points` and by restarting DB.

__Use case:__
It would be a big advantage if this value can be changed depending on the amount of data DB can handle during one request.

Start by reading the existing max-select-points configuration behavior and the linked issue #5323 for the proposed session-level approach. Define how the value should be supplied per session and confirm that changing it takes effect without restarting the database; the issue is complete when that behavior is specified and implemented.

Written by the indexing model from the issue text.