[lldb-server] Update tests to use std::thread/mutex for all platforms

Summary:
Some cleanup suggested when bringing up lldb-server on Windows. 
Thanks to Hui Huang for the patch.

Reviewers: zturner, labath, jfb, Hui

Reviewed By: labath

Subscribers: clayborg, dexonsmith, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D60496

llvm-svn: 358265