Create explicit immutable table states: enabled/disabled

This task is to create a new explicit immutable table state, "enabled" vs. "disabled". When a table is enabled, all normal operations are available to it. When it is disabled, only certain specific operations are available that guaranteed to be non-destructive, table-immutable, operations. Specifically, the "clone" and "export" operations for a table are appropriate for the disabled state.

These states should replace "online" and "offline" states in future versions, particularly in the elasticity branch, where tablets can have a kind of "online" and "offline" state with the tablet availability. However, this could be implemented now, prior to the elasticity branch, to ease the transition.

No files, tests, or entry points are named. Start by locating the existing table "online" and "offline" state handling, then trace the operations allowed in each state. Done means explicit immutable "enabled" and "disabled" states replace those names, with only clone and export available while disabled, and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.