utxonursery: add new public NurseryReport method
utxonursery: add new public NurseryReport method

This commit adds a new public facing method to the utxoNursery:
NurseryReport. This method is intended to be used by callers to
introspect into the current state of the utxo nursery. With this
information, callers will be able to ascertain: the number of immature
outputs, the total number of funds in limbo, and when funds can be
fully swept.

ddustin/lnd,aakselrod/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd