API for Okapi timer task management

Currently, the only way to "disable" an Okapi timer task or to change its run interval is to POST a modified module descriptor for the affected module with an updated _timer interface, POST a deployment descriptor to route the requests to the right module instance, and enable the modified module for the tenant (with ?invoke=false to avoid calling the _tenant interface). This is a little bit error-prone, and makes upgrades tricky, as well. It would be very convenient to have an API for managing tasks that are registered via the Okapi _timer system interface.

The API would probably need to define a new descriptor (a "timer descriptor"), and might be defined thus:

GET /_/proxy/tenants/{tenantId}/timers: get a list of all registered timer descriptors for the tenant
GET /_/proxy/tenants/{tenantId}/timers/{id}: get a registered timer descriptor
PUT /_/proxy/tenants/{tenantId}/timers/{id}: put an updated timer descriptor

One of the fields in the timer descriptor might be something like status with an enum of ["enabled","disabled"] to allow for temporarily disabling a timer.

I leave it up for discussion if the API should allow POST and DELETE. Also up for discussion is what should happen on module upgrade.