Convert Spanner to use Watchdog from GAX #2999
Description
Deprecate https://github.com/GoogleCloudPlatform/google-cloud-java/blob/master/google-cloud-spanner/src/main/java/com/google/cloud/spanner/spi/v1/WatchdogInterceptor.java , use https://github.com/googleapis/gax-java/blob/master/gax/src/main/java/com/google/api/gax/rpc/Watchdog.java instead.
If Spanner uses watchdogs for client streaming or bidi streaming cases, then analogs to https://github.com/googleapis/gax-java/blob/master/gax/src/main/java/com/google/api/gax/rpc/WatchdogServerStreamingCallable.java will need to be created in GAX.