Add warning threshold for excessively large number of channels per user

We already have warning thresholds to nudge customers away from potential issues.
Those existing today are:

• xattrr size limits (90% of 1MB)
• channels per doc (50)
• access grants per doc (50)

We could also add a new warning about channel count per user, to give early warning about running into memory issues with extremely high numbers of channels per user.

The expected limits of channels per user are higher than the ~50 we expect to see on a single doc.

Warning at >1000 channels per user seems to be a reasonable balance, but open to suggestions.