lnd: fix build error for go 1.6.3, use experimental context import
lnd: fix build error for go 1.6.3, use experimental context import

This commit fixes a build error for go 1.6.3. Before go 1.7, the
context package was under the experimental go packages, therefore to
maintain compatibility across our currently supported versions, we must
use the ‘/x/net’ import path.

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