ContentDbPlugin.py - loadFilesOptional - KeyError: u'siteaddress'

### Step 1: Please describe your environment

* ZeroNet version: 3853
* Operating system: Linux
* Special configuration: Bootstrapper plugin, multiuser local plugin; Python 2.7.12

### Step 2: Describe the problem:

When i try to download any site:
python ../zeronet.py --silent siteDownload "15mQd6kBRzriaVEjdELujex2YKFfWaA8JQ"

it returns this:

```
- Unhandled exception
Traceback (most recent call last):
File "src/gevent/greenlet.py", line 766, in gevent._greenlet.Greenlet.run
File "/root/ZeroNet-master/plugins/OptionalManager/ContentDbPlugin.py", line 97, in loadFilesOptional
site = self.sites[site_address]
KeyError: u'1GrwWFTmrKgUGQsGxrKSsRbb2dt8UJaBPN'
Traceback (most recent call last):
File "src/gevent/greenlet.py", line 766, in gevent._greenlet.Greenlet.run
File "/root/ZeroNet-master/plugins/OptionalManager/ContentDbPlugin.py", line 97, in loadFilesOptional
site = self.sites[site_address]
KeyError: u'1GrwWFTmrKgUGQsGxrKSsRbb2dt8UJaBPN'
```

The KeyError is always about site "1GrwWFTmrKgUGQsGxrKSsRbb2dt8UJaBPN".

The memory usage rise like 100MB or more due to siteDownload command and as a result the primary zeronet that is running crash. Also i see 2 same siteDownload processes for same site at same time, which is strange, then like 30 seconds later i see 5+ same processes of siteDownload (for same site) showing like 40% RAM usage. Then the siteDownload command gets killed.