eap: Keep track of trimmed attributes

[loewenheim]

When attributes are deleted by the EAP trimming processor, we need to make a note of the removed attributes. However, since attribute keys are open-ended and can be arbitrarily big, we can't just write them into meta unconditionally. There needs to be a limit to protect our infrastructure, probably in the form of a budget for retained attribute keys that is large enough not to run out unless someone is griefing us.

[linear]

RELAY-196