Improve defaults in Swarm.ConnMgr

[lidel]

• switch Swarm.ConnMgr config from hardcoded strings to optional types:

  *OptionalString  `json:",omitempty"` // type
  *OptionalInteger `json:",omitempty"` // low|high water
  *OptionalDuration `json:",omitempty"` // graceperiod

  • this allows us to be backward-compatible, but remove default values from user config, enabling us to adjust implicit default over time
  • prior art – see how we use fields from Swarm.RelayService
• (TBD) change the defaults to something lower (prior art: ipfs-desktop – but we can discuss new implicit defaults on the PR, and decide to not change them for now)

[lidel]

@schomatis i think you may add this to your queue (not urgent, but useful to fix)