don't set HeartbeatEmitter to nil in dropsonde.Initialize

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>
Removing the unused parameter origin from the BeginHeartbeat signature

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>
Remove getters/setters for HeartbeatInterval and fix tests to clean up
properly to avoid race condition

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>
log error when heartbeat emit fails

Signed-off-by: Johannes Petzold <jpetzold@pivotallabs.com>