Fixes LogEvent constructor to use reference.

Previously when fixing LogEvent missing strings because of pointer
lifetime issues, I had the constructor for LogEvent pass log_msg by
value. This fixes log_msg to pass by reference again so that we avoid
wasting memory.

Test: Checked statsd unit-tests still pass.
Change-Id: I94ac4d1bed19fec19d44e27fe94e07ab30383363