Change RequestSynchronousCheckpoint to release thread_list_lock_

The RequestSynchronousCheckpoint function in some cases needs to
release the thread_list_lock_ as it waits for a checkpoint to be
executed. This means that the thread being checkpointed might be
deleted. Previously it was not obvious this was the case since the
thread_list_lock_ seemed to be held throughout the execution of the
method.

In order to prevent bugs we make RequestSynchronousCheckpoint
explicitly release the thread_list_lock_ when executed, meaning code
will be aware that threads might die during its execution.

Bug: 67362832
Test: ./test.py --host -j50
Change-Id: I1cbdf7660096dc1908b0eeabc1062447307bc888