DefaultDnsResolver in Nima can throw exception

[spericas]

The DefaultDnsResolver used by WebClient is not thread safe and can throw ConcurrentModificationException when a WebClient is used by multiple threads. Needs to use a ConcurrentHashMap.

Caused by: java.util.ConcurrentModificationException
        at java.base/java.util.HashMap.computeIfAbsent(HashMap.java:1229)
        at io.helidon.nima.webclient.DefaultDnsResolver.resolveAddress(DefaultDnsResolver.java:32)
        at io.helidon.nima.webclient.http1.Http1ClientConnection.connect(Http1ClientConnection.java:118)
        at io.helidon.nima.webclient.http1.ClientRequestImpl.getConnection(ClientRequestImpl.java:347)
        at io.helidon.nima.webclient.http1.ClientRequestImpl.submit(ClientRequestImpl.java:158)
        at io.helidon.nima.webclient.http1.ClientRequestImpl.request(ClientRequestImpl.java:145)
        at io.helidon.nima.webclient.http1.ClientRequestImpl.request(ClientRequestImpl.java:58)
        at io.helidon.nima.webclient.ClientRequest.request(ClientRequest.java:135)
        at io.examples.helidon.nima.BlockingService.callRemote(BlockingService.java:109)
        at io.examples.helidon.nima.BlockingService.lambda$parallel$0(BlockingService.java:78)
        at java.base/java.util.concurrent.ThreadPerTaskExecutor$ThreadBoundFuture.run(ThreadPerTaskExecutor.java:352)
        ... 4 more