ENG-2793 Do not fail when deciding if we can flush an empty immutable…

… memtable

Summary:
There was a crash during one of our performance integration tests that was caused by Frontiers() not being set on a memtable. That could only possibly happen if the memtable is empty, and it is still not clear how an empty memtable could get into the list of immutable memtables. Regardless of that, instead of crashing, we should just flush that memtable and log an error message.

```
#0  operator() (memtable=..., __closure=0x7f2e454b67b0) at ../../../../../src/yb/tablet/tablet_peer.cc:178
yugabyte#1  std::_Function_handler<bool(const rocksdb::MemTable&), yb::tablet::TabletPeer::InitTabletPeer(const std::shared_ptr<yb::tablet::enterprise::Tablet>&, const std::shared_future<std::shared_ptr<yb::client::YBClient> >&, const scoped_refptr<yb::server::Clock>&, const std::shared_ptr<yb::rpc::Messenger>&, const scoped_refptr<yb::log::Log>&, const scoped_refptr<yb::MetricEntity>&, yb::ThreadPool*)::<lambda()>::<lambda(const rocksdb::MemTable&)> >::_M_invoke(const std::_Any_data &, const rocksdb::MemTable &) (__functor=..., __args#0=...)  at /n/jenkins/linuxbrew/linuxbrew_2018-01-09T08_28_02/Cellar/gcc/5.5.0/include/c++/5.5.0/functional:1857
yugabyte#2  0x00007f2f7346a70e in operator() (__args#0=..., this=0x7f2e454b67b0) at /n/jenkins/linuxbrew/linuxbrew_2018-01-09T08_28_02/Cellar/gcc/5.5.0/include/c++/5.5.0/functional:2267
yugabyte#3  rocksdb::MemTableList::PickMemtablesToFlush(rocksdb::autovector<rocksdb::MemTable*, 8ul>*, std::function<bool (rocksdb::MemTable const&)> const&) (this=0x7d02978, ret=ret@entry=0x7f2e454b6370, filter=...)
    at ../../../../../src/yb/rocksdb/db/memtable_list.cc:259
yugabyte#4  0x00007f2f7345517f in rocksdb::FlushJob::Run (this=this@entry=0x7f2e454b6750, file_meta=file_meta@entry=0x7f2e454b68d0) at ../../../../../src/yb/rocksdb/db/flush_job.cc:143
yugabyte#5  0x00007f2f7341b7c3 in rocksdb::DBImpl::FlushMemTableToOutputFile (this=this@entry=0x89d2400, cfd=cfd@entry=0x7d02300, mutable_cf_options=..., made_progress=made_progress@entry=0x7f2e454b709e,
    job_context=job_context@entry=0x7f2e454b70b0, log_buffer=0x7f2e454b7280) at ../../../../../src/yb/rocksdb/db/db_impl.cc:1586
yugabyte#6  0x00007f2f7341c19f in rocksdb::DBImpl::BackgroundFlush (this=this@entry=0x89d2400, made_progress=made_progress@entry=0x7f2e454b709e, job_context=job_context@entry=0x7f2e454b70b0,
    log_buffer=log_buffer@entry=0x7f2e454b7280) at ../../../../../src/yb/rocksdb/db/db_impl.cc:2816
yugabyte#7  0x00007f2f7342539b in rocksdb::DBImpl::BackgroundCallFlush (this=0x89d2400) at ../../../../../src/yb/rocksdb/db/db_impl.cc:2838
yugabyte#8  0x00007f2f735154c3 in rocksdb::ThreadPool::BGThread (this=0x3b0bb20, thread_id=0) at ../../../../../src/yb/rocksdb/util/thread_posix.cc:133
yugabyte#9  0x00007f2f73515558 in rocksdb::BGThreadWrapper (arg=0xd970a20) at ../../../../../src/yb/rocksdb/util/thread_posix.cc:157
yugabyte#10 0x00007f2f6c964694 in start_thread (arg=0x7f2e454b8700) at pthread_create.c:333
```

Test Plan: Jenkins

Reviewers: hector, sergei

Reviewed By: hector, sergei

Subscribers: sergei, bogdan, bharat, ybase

Differential Revision: https://phabricator.dev.yugabyte.com/D4044

Note:
This commit provides additional functionality that is logically related to
the earlier commit yugabyte@864e72b
and supersedes the commit yugabyte@2932b0a

ent/src/yb/integration-tests/CMakeLists-include.txt

@@ -1,3 +1,3 @@
 # Copyright (c) YugaByte, Inc.
 
-ADD_YB_TEST(snapshot-test RESOURCE_LOCK "master-rpc-ports")
+ADD_YB_TEST(snapshot-test)

ent/src/yb/integration-tests/snapshot-test.cc

@@ -406,8 +406,8 @@ TEST_F(SnapshotTest, SnapshotRemoteBootstrap) {
 
   workload.StopAndJoin();
 
-  cluster_->FlushTablets();
-  cluster_->CleanTabletLogs();
+  ASSERT_OK(cluster_->FlushTablets());
+  ASSERT_OK(cluster_->CleanTabletLogs());
 
   ASSERT_OK(ts0->Start());
   ASSERT_NO_FATALS(VerifySnapshotFiles(snapshot_id));