Add serialization timeout configurable property

[tamasmak]

• Makes serialization timeout configurable to wait for the serialization to be completed (default timeout has been increased to 30s)
  • If using SpringBoot then a timeout property in SerializationProperties is available to set the timeout
  • If not using SpringBoot then a vaadin.serialization.timeout system property is available to set the timeout
• Updates log messages to make it more clear when a serialization timeout happens and when a serialization job is finished
• Adds check for serialization timeout when updating the result messages in Job::complete to avoid ConcurrentModificationException because in case of timeout the serialization could still run and updating the messages too

Closes #94 and #97

[mcollovati]

Some comments for potential minor changes, but other than that LGTM