chore: rename system property for core threads for Async API (#2657)

Rename the System property that is used to set the number of core
threads for the Async API to a namespaced name to be consistent with
other System properties. The name name is
com.google.cloud.spanner.async_num_core_threads.

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

@@ -530,7 +530,7 @@ static CloseableExecutorProvider createDefaultAsyncExecutorProvider() {
 
   @VisibleForTesting
   static int getDefaultAsyncExecutorProviderCoreThreadCount() {
-    String propertyName = "SPANNER_ASYNC_NUM_CORE_THREADS";
+    String propertyName = "com.google.cloud.spanner.async_num_core_threads";
     String propertyValue = System.getProperty(propertyName, "8");
     try {
       int corePoolSize = Integer.parseInt(propertyValue);

google-cloud-spanner/src/test/java/com/google/cloud/spanner/SpannerOptionsTest.java

@@ -917,7 +917,7 @@ public void testCustomAsyncExecutorProvider() {
   @Test
   public void testAsyncExecutorProviderCoreThreadCount() throws Exception {
     assertEquals(8, SpannerOptions.getDefaultAsyncExecutorProviderCoreThreadCount());
-    String propertyName = "SPANNER_ASYNC_NUM_CORE_THREADS";
+    String propertyName = "com.google.cloud.spanner.async_num_core_threads";
     assertEquals(
         Integer.valueOf(8),
         runWithSystemProperty(