Slow start due to quadratic time algorithms

### Step 1: Please describe your environment

* ZeroNet version: v0.7.2 (rev4555)
* Operating system: N/A
* Web browser: N/A
* Tor status: N/A
* Opened port: N/A
* Special configuration: N/A

### Step 2: Describe the problem:

ZeroNet takes about an hour to start with a few thousand sites downloaded.

The logs say:

```
[13:03:17] SiteManager Added new site: [address]
[13:03:17] Site:1A28C5..E1dw ContentDb init: 0.000s, found files: 0, sites: 8963
[13:03:17] Site:1A28C5..E1dw ContentDb not initialized, load files from filesystem...
[13:03:17] Site:1A28C5..E1dw Content.json not exist: ./data/[address]/content.json
[13:03:17] RateLimit Added to queue (2.75s left): Save sites.json
[13:03:17] SiteManager Updated merger sites in 0.601s
```
(repeat a few thousand times)

The 'Updated merger sites' seems to be the culprit. It's from `SiteManagerPlugin.updateMergerSites`, which scans all the downloaded sites, and is invoked every time a site is initialized. That takes quadratic time.

#### Steps to reproduce:

1. Create/download lots of sites
2. ???
3. PROFIT

#### Observed Results:

Quadratic time intiailization.

#### Expected Results:

Linear (ideally) time initialization.