Make uniform submaps Copy On Write

[kevingranade]

Is your feature request related to a problem? Please describe.
We waste a non-trivial amount of memory because "uniform" submaps each contain a copy of the submap data, even though it's totally redundant.

Describe the solution you'd like
Establish a cache of immutable uniform submaps.
When a uniform submap is generated, check for a match in the cache, if present just use that pointer, if not, insert a copy of the submap and capture the pointer.
If a uniform submap is written to, replace it with a copy of the uniform submap before writing to it.

[Maleclypse]

Closed by #63762