lntest: add new LookUpNodeByPub method to lookup active nodes by pubkey
lntest: add new LookUpNodeByPub method to lookup active nodes by pubkey

In this commit, we add a new helper function to the NetworkHarness
struct. This helper function serves to allow test authors to look up
pointer to an active node based on its current public key.

Each time a new node is started, its public key will be re-registered
within the global nodesByPub map.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd