Admit memory not set in backend descriptor for coordinator-only nodes

When configuring admission control with dedicated coordinator daemons, queries in pools with memory limits fail with the admission rejections like the following:

Rejected query from pool root.default: request memory needed 3.00 GB per node is greater than memory available for admission 0 of coord1.example.com:22000. Use the MEM_LIMIT query option to indicate how much memory is required per node.

Tracing this in the code leads us to line 576 of admission-controller.cc and therefore to suspect that the local TBackendDescriptor (local_backend_descriptor_) for the coordinator node in scheduler.cc never has admit_mem_limit set, and thus ends up with the default value of 0.

The issue goes away if NO_SPECIALIZATION is used instead of COORDINATOR_ONLY.