Limit unreliable vertexes uncached per frame.

Remember them longer, and avoid slamming too many in the same frame.
Refactor vertex caching a little.
Use a more frequent mini hash in the vertex cache.
Avoid full rehashes when failures are frequent.
Vertex cache a bit more frequently.

Rely upon the minihashing a bit more.
Merge pull request #6855 from unknownbrackets/vertexcache

Use a minihash in the vertex cache, treat unreliable arrays more carefully