Fixes sbt/sbt#8142

This updates SbtUpdateReport.caching to use weak references for both the cached keys and values to prevent memory leaks.

Keys

Weak references to keys are handled by using a WeakHashMap instead of a ConcurrentHashMap.

Note that this also required passing the map to Collections.synchronizedMap to ensure it's still thread-safe. I'm guessing that a goal of sbt-coursier is to include as few dependencies as possible, but if that's not correct then I think it's likely that a better alternative to Collections.synchronizedMap would be to use one of:

1. Guava's MapMaker class with calls to weakKeys and weakValues
2. A Caffeine cache with calls to weakKeys and weakValues

Values

Weak references to values are handled by wrapping each value in WeakReference.

Results

Using https://github.com/mrdziuban/sbt-update-memory-leak, I ran:

./run.sh 500
./run.sh 1000
./run.sh 2000

and got the following results for the heap retained by SbtUpdateReport$: