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.

Start by reproducing the siteDownload command with the reported ZeroNet version and inspect plugins/OptionalManager/ContentDbPlugin.py at loadFilesOptional line 97. Trace why the requested site address is absent from self.sites and why duplicate downloads accumulate. Done means the KeyError and duplicate processes no longer occur and siteDownload does not cause the reported memory growth or crash.

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