In eclipse#1103, it was noted that
LanguageServerWrapperTest#testStopAndActivate causes OOM errors. On
investigation, it was discovered that there are multiple causes for
this.

1) The MockConnectionProviderMultiRootFolder used in the test created a
message processor thread in LSP4J on each call to start() but did not
shut them down.
2) The termination of the start/stop loop in the test was wrong: the
loop ran for as long as the VM running tests was alive.
3) The "already stopping" logic in LanguageServerWrapper#shutdown was
wrong: while the shutdown of a LanguageServerWrapper was processed,
further calls to shutdown were ignored.
4) There was a race between the initializationFuture of the
LanguageServerWrapper and the future used for shutdown.

With this commit, the test is rewritten to avoid crashes and to properly
test that calling stop / start repeatedly on LanguageServerWrapper does
not leave any connection providers running. The LanguageServerWrapper
class is refactored to make the test pass.