Tmreal (#12)

* Memtree (#11)

Storing intermidiary IAVL versions in memory and not to disk

Motivation: Both Cosmos and Loom Network save an IAVL version per block, then go back and delete these versions. So you have constant churn on the IAVL and underlying Leveldb database. When realistically what you want is to only store every X Blocks.

At Berlin Tendermint Conference, Zaki and I surmised a plan where new versions are in memory, while still pointing back to nodes on disk to prevent needing to load entire IAVL into main memory. Loom IAVL tree is around 256gb so this is not feasible otherwise.


Usage

OLD Code would be like

```go
hash, version, err := s.tree.SaveVersion()
```

New Caller code would look like

```go
	oldVersion := s.Version()
  	var version int64
 	var hash []byte
 	//Every X versions we should persist to disk
 	if s.flushInterval == 0 || ((oldVersion+1)%s.flushInterval == 0) {
 		if s.flushInterval != 0 {
 			log.Error(fmt.Sprintf("Flushing mem to disk at version %d\n", oldVersion+1))
 			hash, version, err = s.tree.FlushMemVersionDisk()
 		} else {
 			hash, version, err = s.tree.SaveVersion()
 		}
 	} else {
 		hash, version, err = s.tree.SaveVersionMem()
 	}
```

FlushMemVersionDisk:
Flushes the current memory version to disk

SaveVersionMem:
Saves the current tree to memory instead of disk and gives you back an apphash

This is an opt in feature, you have to call new apis to get it. 
We also have a PR that demonstrates its usage https://github.com/loomnetwork/loomchain/pull/1232/files

We are now commiting every 1000 blocks, so we store 1000x less. Also we have signficant improves in IO at least double from not having to Prune old versions of the IAVL Tree

* Add version saving to in memory, and ability to flush to disk periodically

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Pending
+
+- [IAVL] Ability to store temporary versions in memory only, and flush to disk every X versions. This should greatly reduce IO requirements and disk storage. (@mattkanwisher Loom Network)
+
 ## 0.12.2 (March 13, 2019)
 
 IMPROVEMENTS