Use a CREAM cache for valid witnesses, not pdict

[Vagabond]

CREAM will evict naturally based on idle time and size, so use it to replace the pdict cache that was never collected and would leak memory.

[PaulVMo]

Go a little bigger on the max size? Aspirationally if we have have 1000 PoC per block (3x current rate) and conservatively if epochs are a little long at 50 blocks, we could easily need 50,000 items in the cache.