bgpd, doc, ldpd, lib, tests, zebra: LM fixes

bgpd, doc, ldpd, lib, tests, zebra: LM fixes

Corrections so that the BGP daemon can work with the label manager properly
through a label-manager proxy. Details:

- Correction so the BGP daemon behind a proxy label manager gets the range
  correctly (-I added to the BGP daemon, to set the daemon instance id)
- For the BGP case, added an asynchronous label manager connect command so
  the labels get recycled in case of a BGP daemon reconnection. With this,
  BGPd and LDPd would behave similarly.

Signed-off-by: F. Aragon <paco@voltanet.io>