schema: Memory thresholds should never be exactly 0.0. (#7458)

In the config, memory thresholds such as `distributed.worker.memory.terminate` should never be exactly `0.0`.
Instead, config should use `false` to disable memory management.

This one bit me recently.  My older dask config files used `0.0` to disable the memory management features.  That worked because older versions of `distributed` interpreted the value `0.0` to be the equivalent to `false` for these fields.  But in newer versions, only `false` works.  (I suspect the change occurred in #5904.)

Nowadays, if the config says `0.0`, then `distributed` interprets that literally -- and no memory can be used at all without incurring the wrath of the memory manager!

An easy "fix" is to disallow `0.0` in the user's config.  In json schema, `exclusiveMinimum: 0` ensures that the value `0.0` itself is not permitted by the schema.

distributed/distributed-schema.yaml

@@ -541,15 +541,15 @@ properties:
 
               transfer:
                 oneOf:
-                  - {type: number, minimum: 0, maximum: 1}
+                  - {type: number, exclusiveMinimum: 0, maximum: 1}
                   - {enum: [false]}
                 description: >-
                   When the total size of incoming data transfers gets above this amount,
                   we start throttling incoming data transfers
 
               target:
                 oneOf:
-                  - {type: number, minimum: 0, maximum: 1}
+                  - {type: number, exclusiveMinimum: 0, maximum: 1}
                   - {enum: [false]}
                 description: >-
                   When the process memory (as observed by the operating system) gets
@@ -558,7 +558,7 @@ properties:
 
               spill:
                 oneOf:
-                  - {type: number, minimum: 0, maximum: 1}
+                  - {type: number, exclusiveMinimum: 0, maximum: 1}
                   - {enum: [false]}
                 description: >-
                   When the process memory (as observed by the operating system) gets
@@ -568,7 +568,7 @@ properties:
 
               pause:
                 oneOf:
-                  - {type: number, minimum: 0, maximum: 1}
+                  - {type: number, exclusiveMinimum: 0, maximum: 1}
                   - {enum: [false]}
                 description: >-
                   When the process memory (as observed by the operating system) gets
@@ -577,7 +577,7 @@ properties:
 
               terminate:
                 oneOf:
-                  - {type: number, minimum: 0, maximum: 1}
+                  - {type: number, exclusiveMinimum: 0, maximum: 1}
                   - {enum: [false]}
                 description: >-
                   When the process memory reaches this level the nanny process will kill