build: create parallel travis builds to isolate race condition tests

This commit modifies the travis build script, and our local test script
to ensure that the race condition builds are conducted in a parallel
build. After this commit two travis builds will be kicked off for each
push/commit: one that runs the race condition tests in isolation, and
another that runs the integration tests then the coverage tests.

In order to do the above cleanly, the integration tests are now guarded
behind a build flag. In order to run the integration tests, one now
needs to specify the `-tags rpctest` flag when running the `go test`
command.

lnd_test.go

@@ -1,3 +1,5 @@
+// +build rpctest
+
 package main
 
 import (