feat: RandomStrategy AliveOnly

[mkmark]

Address #1841.

New optional settings to strategy random: aliveOnly and observerTag.

If aliveOnly == true, enable the strategy to choose only from alive outbounds. Outbounds are considered alive unless proven otherwise (by observatory).

Very useful when one has several unstable outbounds, this improves reliability by making these outbounds fail-safe to each other.

Also useful when one wants to combine bandwidth of several unstable outbounds.

Use like

{
  "selector": [
    "a",
    "b"
  ],
  "tag": "c",
  "strategy": {
    "type": "random",
    "settings": {
       "aliveOnly": true
    }
  }
}

Need observatory.

FAQ

How much does this affect the original random strategy?

Fully compatible with the original random strategy, i.e. no strategy at all.

It adds a check upon picking an outbound, i.e. whether ones has a random strategy. Almost no performance effect.

obsolete

New balancer strategy: `randomalive`.

Random, but only choose from alive outbounds.

Use like

{
  "selector": [
    "a",
    "b"
  ],
  "tag": "c",
  "strategy": {
    "type": "randomalive"
  }
}

[xiaokangwang]

Thanks for you contribution. I think this merge request is ready to be merged.

[xiaokangwang]

This pull request have been merged. Please send a pull request to the document if you are willing and able.