Allow explicit management of tablet creation/deletion

This is a feature that has been requested by Xiaomi for some time. There are a couple parts:

1) Allow a table to be created whose tablets don't span the whole key range. This probably has an API change involved.
2) Teach the client to deal with the case where an operation touches a key which falls outside of any tablets. Initially, we can reject such operations.
3) Add admin APIs to explicitly create/drop tablets, or convert tablets to tables.
4) Lower priority: add a facility by which a "catch-all" table can be defined which receives inserts/updates which failed to match any tablet in a table.

Manual partition management is a reasonable way to manage time series workloads in particular.