WIP: Implement writes series data on database

Not ready yet, just opening a PR to start some discussion.

The interface for interacting with the metastore feels clunky on my first pass trying to use it. I almost feel like there needs to be another abstraction over those transactions.

For example, this spot https://github.com/influxdb/influxdb/compare/write_series?expand=1#diff-320bbef96d874515320f176727a55ea8R410 where I have to close over the variables and set them feels really off.

Thoughts @benbjohnson?
Is this two-level use of buckets the final design? Or something you're just trying out? I was expecting to simply see one bucket for "series" and then key and key in the form, for example:

cpuload
cpuload.host.server1

as opposed to a bucket for "series", then a bucket for "cpuload", with a final key for the tags. Is there an advantage to the two-level bucketing?