Revert "llvm dep 3.1 -> 3.3"

This reverts commit 72cff6ad7a10b7091f2f09163ac24de7e3570e4e.
[base] Base64: fixes crash
tests update
mute warning
fixes std::string(cptr, [offset?,] size) ctor use
[http] Vary: update
Vary test update
fix old GCC's...
[http] HttpConnection: added the ability to switch between explicit and auto-flush on writes defaulting to auto-flush.
[capi] expose response writing autoflush
[capi] expose API to post a function+userdata into request's worker thread for execution.
[capi] API improved
[http] HttpWorker: set custom thread names to something more descriptive (/fancy)
[capi] expose an API to allow setting up multiple I/O workers