This commit adds a new feature to the network harness enabling callers
to receive async notifications once a particular transaction is seen on
the network. Such a feature is useful when due to the asynchronous
behavior of node communications.
With this new feature, tests can now wait for a particular transaction
to be seen within the network before proceeding.
This commit fixes the build for version 1.6.3 of golang. In go 1.7, the
“context” package was moved into the standard library, however go 1.6.3
doesn’t have that change, so we must refer to the prior WIP package
until a new version of Go is released.