[FLINK-10580] Harden BootstrapTool#startActorSystem

[tillrohrmann]

What is the purpose of the change

Instead of opening a socket to check whether a given port is free we simply start an
ActorSystem with it an check whether it can bind to this port. This also solves the problem
that a 0 port get resolved to a specific port which might get taken between closing the
test socket and starting the ActorSystem.

Verifying this change

• Added BootstrapToolsTest#testConcurrentActorSystemCreation which failed without this change (non-deterministically)

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[zentol]

+1, great that we simplify this code, I always found it to be a little suspect.

[tisonkun]

+1 great change!

[tillrohrmann]

Thanks for the review @zentol and @tisonkun. Merging this PR.