issue#4364: try to fix memory leak for daemon run.

[suzp1984]

This PR is try to workaround #4364. The memory continues increase problem didn't appear in the PR, but the root cause is not yet founded, by moving global initialize code into srs_pre_global_initialize(), It seems those 2 instructions may trigger this memory issue.

    _srs_log = new SrsFileLog();
    _srs_context = new SrsThreadContext();

[winlinvip]

It doesn't make sense because they are two variables with fixed memory; they should not cause the memory to increase over time.