Add a CLI flag to override the points-writer's write timeout

[danxmoran]

The DB's points-writer component has a WriteTimeout field which is initialized to a default value of 10 seconds. When users try to load a significant amount of data their requests can surpass this timeout, resulting in errors (see this community post). We don't currently expose a CLI/config option for changing the timeout value, so users in this situation are stuck.

Things TODO:

• Add a CLI flag to support changing the timeout value
• Consider bumping the default timeout