ENG-3189: Fix intermittent "transaction expired" error when committin…

…g a distributed transaction

Summary: In transaction coordinator, the last_touch_ time of a transaction may be skewed compared to the coordinator's clock. Sometimes the last_touch_ time may go after the coordinator's clock and thus an underflow error when last_touch_ time is subtracted from now() in TransactionState::ExpiredAt() and the transaction erroneously considered expired.

Test Plan:
Run CassandraTransactionalKeyValue read/write workload with 1000000 keys inserted in a GCP cluster with no "transaction expired" error.

```
java -jar ~/code/yugabyte/java/yb-loadtester/target/yb-sample-apps.jar -workload CassandraTransactionalKeyValue -num_threads_read 64 -num_threads_write 8 -nodes ...
```

Reviewers: mikhail, sergei

Reviewed By: sergei

Differential Revision: https://phabricator.dev.yugabyte.com/D4615

src/yb/client/ql-transaction-test.cc

@@ -42,7 +42,7 @@
 
 using namespace std::literals; // NOLINT
 
-DECLARE_uint64(transaction_timeout_usec);
+DECLARE_int64(transaction_timeout_usec);
 DECLARE_uint64(transaction_heartbeat_usec);
 DECLARE_uint64(transaction_table_num_tablets);
 DECLARE_uint64(log_segment_size_bytes);

src/yb/tablet/transaction_coordinator.cc

@@ -55,7 +55,7 @@
 #include "yb/util/tsan_util.h"
 
 DECLARE_uint64(transaction_heartbeat_usec);
-DEFINE_uint64(transaction_timeout_usec, 1500000, "Transaction expiration timeout in usec.");
+DEFINE_int64(transaction_timeout_usec, 1500000, "Transaction expiration timeout in usec.");
 DEFINE_uint64(transaction_check_interval_usec, 500000, "Transaction check interval in usec.");
 DEFINE_double(transaction_ignore_applying_probability_in_tests, 0,
               "Probability to ignore APPLYING update in tests.");
@@ -149,7 +149,7 @@ class TransactionState {
     if (ShouldBeCommitted()) {
       return false;
     }
-    auto passed = now.GetPhysicalValueMicros() - last_touch_.GetPhysicalValueMicros();
+    const int64_t passed = now.GetPhysicalValueMicros() - last_touch_.GetPhysicalValueMicros();
     return passed > FLAGS_transaction_timeout_usec;
   }