travis: execute make travis step by step
travis: execute make travis step by step

Instead of calling 'make travis' directly, we call each step. This lets us
better track how much time is spent on each.

Also note that we execute 'itest-only' instead of 'itest', and instead
execute the dependencies (btcd, build) manually first.

We explicitly set the ITEST environment variable, for readability, and
define a new COVER. This is currently true when ITEST=true to keep the
existing build configuration, but will later be configured to be
independent.

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