Update relevant configuration to use optional types

[BigLep]

Now that go-ipfs-config supports optional types, we should go through and clean up as much of the existing config to use it so we have a consistent story on defaults, etc.

This work will be easier to do after ipfs/go-ipfs-config#151

The first step is to enumerate the various config keys we want to cleanup.

[aschmahmann]

Pointers here:

• The config docs show fields like the HighWater in the connection manager being integers instead of optional integers (e.g. Bitswap task workers). We should make our fields optional
• Requires changes in https://github.com/ipfs/go-ipfs-config and in go-ipfs

[hboisgibault]

Hii @aschmahmann, it seems that the HighWater field is correctly set as optional :
https://github.com/ipfs/kubo/blob/master/config/swarm.go

If I understand correctly, all fields that have a default should be optional.

I can first find the list of fields in to change.