This commit re-organizes the static maps we use to perform lookups into the
`UnleasableSystemDescriptors`. It splits the `UnleasableSystemDescriptors` map
into two, one optimized for `leasedDescriptors.getByName` and one optimized for
`leasedDescriptors.getByID`.

In a 100% read workload, `leasedDescriptors` accesses (`getByName` and
`getByID`) were responsible for **4.39** of total CPU utilization.

Within this, about **0.33%** of total CPU utilization was spent directly in
these functions. This change shouldn't make a large difference (less than
**0.2%**), but it should improve things slightly. Since it also improves
readability, it seems worthwhile.

```
      File: cockroach
Type: cpu
Time: Dec 30, 2021 at 10:57pm (UTC)
Duration: 30.15s, Total samples = 74.99s (248.72%)
Active filters:
   focus=leasedDescriptors\).getBy
Showing nodes accounting for 3.29s, 4.39% of 74.99s total
----------------------------------------------------------+-------------
      flat  flat%   sum%        cum   cum%   calls calls% + context
----------------------------------------------------------+-------------
                                             0.14s   100% |   github.com/cockroachdb/cockroach/pkg/sql/catalog/descs.(*Collection).getByName /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descs/descriptor.go:199
     0.10s  0.13%  0.13%      0.14s  0.19%                | github.com/cockroachdb/cockroach/pkg/sql/catalog/descs.(*leasedDescriptors).getByName /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descs/leased_descriptors.go:102
                                             0.03s 21.43% |   github.com/cockroachdb/cockroach/pkg/sql/catalog/descpb.(*TableDescriptor).GetParentID /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descpb/structured.pb.go:2347
                                             0.01s  7.14% |   github.com/cockroachdb/cockroach/pkg/sql/catalog/dbdesc.(*immutable).GetParentID /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/dbdesc/database_desc.go:93
----------------------------------------------------------+-------------
                                             0.14s   100% |   github.com/cockroachdb/cockroach/pkg/sql/catalog/descs.(*Collection).getByName /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descs/descriptor.go:199
     0.04s 0.053%  0.19%      0.14s  0.19%                | github.com/cockroachdb/cockroach/pkg/sql/catalog/descs.(*leasedDescriptors).getByName /go/src/github.com/cockroachdb/cockroach/pkg/sql/catalog/descs/leased_descriptors.go:101
                                             0.03s 21.43% |   runtime.mapiternext /usr/local/go/src/runtime/map.go:851
                                             0.02s 14.29% |   runtime.mapiterinit /usr/local/go/src/runtime/map.go:821
                                             0.01s  7.14% |   runtime.duffzero /usr/local/go/src/runtime/duff_amd64.s:95
                                             0.01s  7.14% |   runtime.mapiterinit /usr/local/go/src/runtime/map.go:832
                                             0.01s  7.14% |   runtime.mapiterinit /usr/local/go/src/runtime/map.go:848
                                             0.01s  7.14% |   runtime.mapiternext /usr/local/go/src/runtime/map.go:898
                                             0.01s  7.14% |   runtime.mapiternext /usr/local/go/src/runtime/map.go:972
----------------------------------------------------------+-------------
```