When Discontinuity seq in WAL, there is too much unuse read wal file in the nextimpl

We use wal to sync data between master and slave. So we always first call GetUpdatesSince(seq, &iter), then call NextImpl to iterate the master data, finally send the data to slave. But when we write another data to master with 'disableWAL = true', our sync-thread is always do below call stack with little data to sync. It means too much read disk but too little syncing. It leads wal block.
rocksdb version is 5.7.2
```
`Thread 1 (Thread 0x7f67e3def700 (LWP 25245)):
#0 rocksdb::crc32c::ExtendImpl (crc=0, buf=, size=) at util/crc32c.cc:356
#1 0x0000000000945d7b in rocksdb::crc32c::Value (n=, data=) at ./util/crc32c.h:26
#2 rocksdb::log::Reader::ReadPhysicalRecord (this=this@entry=0x220b15a00, result=result@entry=0x7f67e3decff0, drop_size=drop_size@entry=0x7f67e3decfe8) at db/log_reader.cc:405
#3 0x0000000000945eac in rocksdb::log::Reader::ReadRecord (this=0x220b15a00, record=, scratch=0x7f67e3ded100, wal_recovery_mode=) at db/log_reader.cc:95
#4 0x0000000000acbf3e in rocksdb::TransactionLogIteratorImpl::SeekToStartSequence (this=this@entry=0x234aa32c0, startFileIndex=, strict=strict@entry=false) at db/transaction_log_impl.cc:110
#5 0x0000000000acc6f9 in rocksdb::TransactionLogIteratorImpl::UpdateCurrentWriteBatch (this=this@entry=0x234aa32c0, record=...) at db/transaction_log_impl.cc:245
#6 0x0000000000acc9ab in rocksdb::TransactionLogIteratorImpl::NextImpl (this=0x234aa32c0, internal=) at db/transaction_log_impl.cc:180
#7 0x0000000000798b03 in nextIterator (rocksWalLogIterator=0x25117c0a0) at /root/project/fusion.r2/src/rocks_wallog.cpp:69
#8 0x0000000000799123 in RocksWalLogIterator::next (this=0x25117c0a0) at /root/project/fusion.r2/src/rocks_wallog.h:61
#9 rocksWallogAction (sync_msg=sync_msg@entry=0x7f67e3ded5d0, rocksWalLogIterator=0x25117c0a0, data=data@entry=0x2093c9fc0, lastOffset=lastOffset@entry=0x7f67e3ded520, pack_size=pack_size@entry=204800, error=...) at /root/project/fusion.r2/src/rocks_wallog.cpp:281
#10 0x0000000000802742 in repl_operate () at /root/project/fusion.r2/src/repl.cpp:708
#11 0x00000000008038f0 in write_slave (bev=0x22a018b40, ctx=) at /root/project/fusion.r2/src/repl.cpp:875
#12 0x0000000000b554f9 in bufferevent_trigger_nolock_ (options=0, iotype=4, bufev=0x22a018b40) at bufferevent-internal.h:397
#13 bufferevent_writecb (fd=, event=, arg=0x22a018b40) at bufferevent_sock.c:323
#14 0x0000000000b5b4d0 in event_persist_closure (ev=, base=0x20a243180) at event.c:1580
#15 event_process_active_single_queue (base=base@entry=0x20a243180, activeq=0x3318ae0, max_to_process=max_to_process@entry=2147483647, endtime=endtime@entry=0x0) at event.c:1639
#16 0x0000000000b5becf in event_process_active (base=0x20a243180) at event.c:1738
#17 event_base_loop (base=0x20a243180, flags=flags@entry=0) at event.c:1961
#18 0x0000000000b5c127 in event_base_dispatch (event_base=) at event.c:1772
#19 0x00000000007fc714 in sync_port_loop (data=) at /root/project/fusion.r2/src/repl.cpp:1824
#20 0x0000000000b33465 in g_thread_proxy (data=0x220b4d000) at gthread.c:778
#21 0x00007f685b6f3dc5 in start_thread () from /lib64/libpthread.so.0
#22 0x00007f685aaf921d in clone () from /lib64/libc.so.6`
```