rocksdb: minor tweaks (#613)

* drafts are writable

* session overwrite should emit events

index.js

@@ -396,6 +396,7 @@ class Hypercore extends EventEmitter {
   }
 
   _isWritable () {
+    if (this.draft) return true
     return !this._readonly && !!(this.keyPair && this.keyPair.secretKey)
   }
 

lib/session-state.js

@@ -499,7 +499,14 @@ module.exports = class SessionState {
     await this.mutex.lock()
 
     try {
-      await this._overwrite(state, length, treeLength, null)
+      const origLength = this.tree.length
+
+      const tree = await this._overwrite(state, length, treeLength, null)
+
+      const bitfield = { start: treeLength, length: tree.length - treeLength, drop: false }
+
+      if (treeLength < origLength) this.ontruncate(tree, treeLength, origLength)
+      if (treeLength < tree.length) this.onappend(tree, bitfield)
 
       return {
         length: this.tree.length,