adds support for limits on the number of tokens per owner

[shamsimam]

Changes proposed in this PR

• adds support for limits on the number of tokens per owner

Why are we making these changes?

Allows us to manage the stress we can end placing on our key-value store. In addition, it prevents rogue processes from creating many tokens.

[DaoWen]

I think we should hold off on this until we have good answers to all of these questions:

1. Can users easily obtain a human-readable list of their tokens? This should be human-readable, and for just one account.
2. Do users actually know how to delete tokens?
3. What happens if this limit is hit during cross-cluster token synchronization?

[shamsimam]

@DaoWen

Can users easily obtain a human-readable list of their tokens? This should be human-readable, and for just one account.

This was not previously publicized but can be obtained from the /tokens?owner-<owner> endpoint. I have added this entry to our REST api documentation (rest-api.md).

Do users actually know how to delete tokens?

We have supported token delete and have it documented in our REST api documentation (rest-api.md).

What happens if this limit is hit during cross-cluster token synchronization?

I have updated the PR to avoid validating the limits during admin mode updates.

[DaoWen]

I don't think the "human friendly" aspects of points 1 and 2 in my earlier feedback have been addressed. I think it would be good to wait until David's project is done before merging this. (But we can still review in the meantime.)

[shamsimam]

@DaoWen Okay, please go ahead with the review.

[shamsimam]

Build is green.