Need threadsafe accesses to DSLinkProvider running field

[muzzle2023]

The rest of the fields in DSLinkProvider are final but running is a plain boolean. Accesses to the running field from start, stop, sleep are not thread safe.

sdk-dslink-java/sdk/dslink/src/main/java/org/dsa/iot/dslink/DSLinkProvider.java

Line 34 in 40dab17

private boolean running;