[Bug] StackerDB configuration can be changed at any point in a tenure

[jcnelson]

When originally designed, StackerDB configurations were not meant to be changed except for at the start of a tenure. This is because the cost of reloading them is borne by the p2p thread and is expensive. Furthermore, the ones for miners and signers don't change very often at all -- no more than once per tenure.

The code, however, does not enforce this. This should get fixed ASAP so that (1) the p2p thread only needs to refresh StackerDB views when the Stacks tenure changes, and (2) before other projects start using StackerDBs in the wild.