[#12853] DocDB: [PITR] Turn consistent_restore flag on by default

Summary:
In order to provide consistency guarantees if PITR restore is executed concurrently with transactions updating the data, the recommendation is to stop application workloads before initiating the restore.

There is a `consistent_restore` flag (introduced in #8419) that provides these guarantees. However, the flag is not enabled by default as it needed to be thoroughly tested. Now that the flag has been validated by QA, this diff sets the default value of `consistent_restore` to true.
Jira: DB-2615

Test Plan:
QA has validated `consistent_restore` flag. Unit tests where the flags is enabled:

ybd --gtest_filter YbAdminSnapshotScheduleTest.ConsistentRestore
ybd --gtest_filter YbAdminSnapshotScheduleTest.ConsistentTxnRestore

Reviewers: zdrudi, skedia

Reviewed By: zdrudi

Subscribers: ybase, bogdan, slingam

Differential Revision: https://phorge.dev.yugabyte.com/D26681

src/yb/tablet/operations/snapshot_operation.cc

@@ -26,8 +26,8 @@
 
 using std::string;
 
-DEFINE_UNKNOWN_bool(consistent_restore, false,
-    "Whether to enable consistent restoration of snapshots");
+DEFINE_UNKNOWN_bool(
+    consistent_restore, true, "Whether to enable consistent restoration of snapshots");
 
 DEFINE_test_flag(bool, modify_flushed_frontier_snapshot_op, true,
                  "Whether to modify flushed frontier after "

src/yb/tools/yb-admin-snapshot-schedule-test.cc

@@ -3223,9 +3223,7 @@ void YbAdminSnapshotScheduleTest::TestGCHiddenTables() {
 
 class YbAdminSnapshotConsistentRestoreTest : public YbAdminSnapshotScheduleTest {
  public:
-  virtual std::vector<std::string> ExtraTSFlags() {
-    return { "--consistent_restore=true", "--TEST_tablet_delay_restore_ms=0" };
-  }
+  virtual std::vector<std::string> ExtraTSFlags() { return {"--TEST_tablet_delay_restore_ms=0"}; }
 };
 
 Status WaitWrites(int num, std::atomic<int>* current) {
@@ -3490,10 +3488,6 @@ TEST_F_EX(YbAdminSnapshotScheduleTest, DDLsDuringRestore, YbAdminSnapshotConsist
 
 class YbAdminSnapshotConsistentRestoreFailoverTest : public YbAdminSnapshotScheduleTest {
  public:
-  std::vector<std::string> ExtraTSFlags() override {
-    return { "--consistent_restore=true" };
-  }
-
   std::vector<std::string> ExtraMasterFlags() override {
     return { "--TEST_skip_sending_restore_finished=true" };
   }