Fix segfault when freed host_upload_ctx is used by xferinfo after free.

This fixes use after free problem when libsx goes into one of the error
paths.
Stop all libcurl transfers when error is SXE_ABORT (bb#672)

Force all pending libcurl transfers to fail if user want to abort. They
are run in parallel, so even when one transfer is stopped with error
code set to SXE_ABORT, then all remaining transfers that were run simultaneously
were continued. This was causing multiple calls to progress
callback even after aborting.
Update libcurl handle speed limit when global limit is reached