feat(spanner): support multiplexed session for read-write transactions (

#3608)

* chore(spanner): update project

* chore(spanner): test update error code and add statements to emptybody

* chore(spanner): test update error code

* chore(spanner): add select 1 for schema refresh time in tests

* chore(spanner): add sleep for cache refresh

* chore(spanner): update error code

* chore(spanner): run it on cloud-devel

* chore(spanner): mutation only aborted case

* chore(spanner): fix conection it tests

* chore(spanner): lint fix

* chore(spanner): enable env

* chore(spanner): skip failing unit tests

* chore(spanner): throw aborted

* chore(spanner): cache miss fix

* chore(spanner): remove temp fixes

* chore(spanner): remove temp fixes

* chore(spanner): comment out exception

* chore(spanner): revert begintxn aborted case to verify tests

* chore(spanner): add env for partitioned ops for integration tests

* chore(spanner): run .write only if there are any mutations to commit

* chore(spanner): skip pops unit tests

* chore(spanner): comment failing unit tests for pops

* chore(spanner): return in interceptor if txn is not created

* chore(spanner): don't throw error if precommit token is empty

* chore(spanner): INVALID_ARGUMENT error code to NOT_FOUND

* chore(spanner): remove mutations only code

* chore(spanner): test fix

* chore(spanner): test fix

* chore(spanner): fix tests

* chore(spanner): lint fix

* chore(spanner): lint fix

* chore(spanner): test fix

* chore(spanner): throw error for resetforretryasync

* chore(spanner): skip test temp

* chore(spanner): tracing test skip

* chore(spanner): tests fix

* chore(spanner): test fix for AsyncRunnerTest

* chore(spanner): remove test skip

* chore(spanner): fix resource cleanup

* chore(spanner): revert cloud-devel to prod

* chore(spanner): lint fix

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

@@ -390,7 +390,7 @@ private static Boolean parseBooleanEnvVariable(String variableName) {
   private static Boolean getUseMultiplexedSessionForRWFromEnvVariable() {
     // Checks the value of env, GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW
     // This returns null until RW is supported.
-    return null;
+    return parseBooleanEnvVariable("GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS_FOR_RW");
   }
 
   Duration getMultiplexedSessionMaintenanceDuration() {