This commit adds a new NextAvailablePortForProcess function that takes a
process ID and then assures unique (non-occupied) port numbers are
returned per process.
This uses a temporary file that contains the latest used port and a
secondary temporary lock file to assure only a single goroutine can
request a new port at a time.
The GenerateProcessUniqueListenerAddresses is intened to be used as a
package-level override for the ListenAddressGenerator variable. We don't
use it by default to make sure we don't break any existing assumptions.
In this commit, we update all the btcutil imports to point to the new
sub-module.
In the same commit, we also modify the recently added `btcutil/go.mod`
file as we need to continue pointing to the _old_ version of btcd, until
we merge this PR and push a new tag.
To allow using a custom btcd executable, we allow specifying a path to a
file. If the path is empty, the harness will fall back to compiling one
from scratch.
Previously, rpctest would start a btcd node using the btcd executable
in the environment PATH. This caused difficult-to-find issues where
the code would be tested against an older version of btcd, or another
fork entirely. Now it compiles btcd the first time it is needed and
uses that fresh version when launching nodes.
