Cache recently used encodings in store

[mikera]

There is a potential significant performance improvement given incoming encodings:

• Keep a array of WeakReference<ACell> in AStore
• Index this by some function of ABlob.toLong()
• Use to lookup Cells already in memory if available
• Write to this array whenever a cell is stored (can overwrite safely on collisions since this is just a cache)

This should help:

• De-duplicate objects in memory
• Avoid decoding encodings if we have already done this
• Avoid hashing for repeatedly encountered encodings (e.g. incoming copies of the same orderings / blocks / transactions from multiple peers)
• Re-use the latest refs with corresponding statuses in the target cells to avoid repeated store checks / writes