feat: limit connections number

[vasco-santos]

In the context of libp2p/js-libp2p-kad-dht#61, the libp2p connections are now managed and the limits are configurable through the config file.

Excerpt from release notes of go-ipfs v0.4.12:

The most critical change is the 'Connection Manager': it allows an ipfs node to maintain a limited set of connections to other peers in the network. By default (and with no config changes required by the user), ipfs nodes will now try to maintain between 600 and 900 open connections. These limits are still likely higher than needed, and future releases may lower the default recommendation, but for now we want to make changes gradually. The rationale for this selection of numbers is as follows:

• The DHT routing table for a large network may rise to around 400 peers
• Bitswap connections tend to be separate from the DHT
• PubSub connections also generally are another distinct set of peers (including js-ipfs nodes)

Because of this, we selected 600 as a LowWater number, and 900 as a HighWater number to avoid having to clear out connections too frequently. You can configure different numbers as you see fit via the Swarm.ConnMgr field in your ipfs config file. See here for more details.

Needs:

• ipfs/js-ipfs#856
• libp2p/js-libp2p-kad-dht#66
• libp2p/js-libp2p#303
• libp2p new release

[alanshaw]

LGTM