TS-2126: Avoid unnecessary memory copy in LogHost::write()

Now, in LogHost::write (LogBuffer *lb), Another new LogBuffer object will be
allocated to copy the content of *lb* parameter.

But *lb* will be deleted by the upper function, LogBufferManager::flush_buffers(),
after LogHost::write() return.

Obviously, the copy is unnecessary, and may cause memory fragmentation.

Let's optimize it.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>