Support selective patching

[tzoiker]

Motivated by #384 - implicit monotonic time patching may (and does) cause hard-to-debug problems in various settings.

• Add Target enum describing functions that can be patched;
• Pass targets parameter to freeze_time to describe functions to be patched;
• Make targets exclude monotonic time by default;
• Add freeze_time_with_monotonic helper to include monotonic time patching.

[Pavkazzz]

@spulec Can we merge this one?
Looks good