Previously SSL handshake timeouts were not properly logged, and resulted
in 502 errors instead of 504 (ticket #1126).

It is not necessary to initialize ctx->ranges in all request, because
ctx->ranges in subrequest will be reassigned to ctx->ranges of main
request.

On Linux, the rename syscall can be slow due to a global file system lock,
acquired for the entire rename operation, unless both old and new files are
in the same directory.  To address this temporary files are now created
in the same directory as the expected resulting cache file when using the
"use_temp_path=off" parameter.

This change mostly reverts 99639bfdfa2a and 3281de8142f5, restoring the
behaviour as of a9138c35120d (with minor changes).

Holding a cache node lock doesn't make sense as we can't use caching
anyway, and results in "ignore long locked inactive cache entry" alerts
if a node is locked for a long time.

The same is done for unbuffered connections, as they can be alive for
a long time as well.

It configures a threshold in bytes, above which client range
requests are not cached.  In such a case the client's Range
header is passed directly to a proxied server.

As the pointer to the first argument was tested instead of the argument
itself, array of arguments was always created, even if there were no
arguments.  Fix is to test args[0] instead of args.

Found by Coverity (CID 1356862).

This is an API change.