Adjust number of connections based on system config

I didn't have this issue before v0.4 and the previous stable Tor, now it makes Tor too busy and connections can't be established. Maybe it's due to the more content on the network or currently poorer quality of my internet connection.
Anyway, I thought to put this out there for discussion: `self.pool` could be set to a lower value for systems with 1GB RAM, medium for 1GB-4GB, and 1,000 for above that. Also the backlog could be adjusted accordingly.

I'm now running v0.4.0 and the latest stable Tor release 0.2.8.6 and getting a lot of this (actual numbers replaced with x).

```
Your Guard x (x) is failing more circuits than usual. Most likely this means the Tor network is overloaded. Success counts are 1xx/1xx. Use counts are 6x/6x. 1xx circuits completed, 5 were unusable, 0 collapsed, and 1x timed out. For reference, your timeout cutoff is xxx seconds..
```

and

```
Tried for 120 seconds to get a connection to [scrubbed]:15441. Giving up.
```

该 issue 将 self.pool 和 backlog 确定为需要检查的设置；首先定位它们在哪里初始化，以及报告的连接失败是如何呈现的。检查系统内存是如何确定的，然后验证三个 RAM 范围的预期限制，并测试是否可以在没有报告的过载情况下建立连接。

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