Fix linux segfault

[SniperJoe]

Problem: on linux, when GetRawZeroPose is called in alvr_server.cpp, the code first checks if OpenVR is initialized. If it is, GetRawZeroPose is called. But in between the check and the call something can call ShutdownOpenvrClient concurrently (and that's what's happening on my machine), and this leads to SEGFAULT because vr::VRCompositor() or vr::VRChaperoneSetup() in GetInvZeroPose returns NULL and the further calls happen on NULL objects.

Solution: add GetRawZeroPose ability to return nullptr if OpenVR is not initialized and do nothing in this case. Also add the same lock to GetInvZeroPose as all the functions above it use.

Solution 2: just an idea and not implemented in this PR. The whole ChaperoneUpdater can be separated to a class and be used atomically/with mutex.

P.S. My previous PR which is not merged yet is included into this one, so it has a bit more code than it should have.

[zmerp]

@SniperJoe now needs rebasing

[galister]

Haven't ran into this myself, but makes sense. Solution 2 would be more proper, but as soon as we get the last piece we need from valve for the direct mode driver, we won't need any of the raw pose hacks, so let's just go with this for now.