Issue #1 : providing a way to shutdown executor services.

[jdoklovic]

While using json-schema-validator in an OSGi environment, we hit some nasty OOMEs due to the executor threads hanging around.

This PR provides a way for us to shut them down when needed without disrupting the library's normal use in non-OSGi envs.

[weintraubt]

Hi. Any chance this PR can be merged soon?

We are validating our JSON Schemas using the json-schema-validator library and are facing this issue since our application is a web application we restart without shutting down the jvm.

Thanks a lot.

[MartinMeinhold]

ping @fge any chance this could be merged? If not could you please kindly let us know what we could do to accelerate this?

[Capstan]

If this is still an issue, can you please port to https://github.com/java-json-tools/msg-simple/issues? This repo is derelict.