channeldb: modify the String() method of ChannelStatus reflect all flags
channeldb: modify the String() method of ChannelStatus reflect all flags

In this commit, we modify the String() method of the ChannelStatus type
to reflect the fact that it's a flag set. With these new changes, we'll
now print the variable name of each assigned bit with a bar delimiting
them all.

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