rocksdb: move MemoryOverlay to storage module (#580)

* add memory overlay test

* fix put

* implement memory overlay block deletion

* add memory overlay block deletion tests

* move tip list to abstraction and add tests

* treeNodes use tip list

* move memory overlay to hypercore-on-the-rocks

* rename peak to peek

lib/core.js

@@ -3,7 +3,6 @@ const b4a = require('b4a')
 const assert = require('nanoassert')
 const unslab = require('unslab')
 const z32 = require('z32')
-const MemoryOverlay = require('./memory-overlay')
 const Mutex = require('./mutex')
 const MerkleTree = require('./merkle-tree')
 const BlockStore = require('./block-store')
@@ -180,7 +179,7 @@ class SessionState {
   }
 
   memoryOverlay () {
-    const storage = new MemoryOverlay(this.storage)
+    const storage = this.storage.createMemoryOverlay()
     const s = new SessionState(
       this.core,
       storage,