Resume support is broken under high concurrency

When building a multi-module project in parallel, Maven updates the local Maven repository without taking into account that other threads or processes might do the same at the same time.

To reproduce:

1. unpack the attached project
2. mvn dependency:list -U -B -T100

See the attached dependency-list.txt for an example output.

First thing to notice is that the dependency is downloaded 100 times. This is unexpected, since the Reactor should coordinate all the modules.

Second thing to notice is that the build fails, since a dependency "cannot be found". This is wrong, since the dependency can be found, it's just not processed properly.

I suspect the legacy.DefaultWagonManager to be the cause of this, since the typical error messages are:

• "Downloaded file does not exist: "
• "Error copying temporary file to the final destination: "
• "*** CHECKSUM FAILED - RETRYING"