Add support for using multiple connections

The -o max_conns=N option causes multiple SSH processes and response processing threads to
be created. This means that e.g. reading a large file no longer blocks all access to the
filesystem.

The connection is chosen by checking the per-connection statistics:

  1. Choose connection with least outstanding requests; if it's a tie,
  2. choose connection with least directory handles; if it's a tie,
  3. choose connection with least file handles; if it's a tie,
  4. choose connection which has already been established.

The implementation assumes that the max_conns values will be small; it
uses linear search.

Example benchmark:

With single connection:

$ sshfs -o max_conns=1,workaround=nobuflimit ebox: mnt
$ cat mnt/tmp/bigfile > /dev/null &
$ time find mnt > /dev/null

real	1m50.432s
user	0m0.133s
sys	0m0.467s

With multiple connections:

$ ~/in-progress/sshfs/build/sshfs -o max_conns=5,workaround=nobuflimit ebox: mnt
$ cat mnt/tmp/bigfile > /dev/null &
$ time find mnt > /dev/null

real	1m15.338s
user	0m0.142s
sys	0m0.491s

This feature was implemented to large extend by Timo Savola <timo.savola@iki.fi>. Thanks
to CEA.fr for sponsoring the remaining work to complete this feature and integrate it into
SSHFS!

ChangeLog.rst

@@ -1,3 +1,10 @@
+Unreleased Changes
+------------------
+
+* New max_conns option enables the use of multiple connections to improve responsiveness
+  during large file transfers. Thanks to Timo Savola for doing most of the implementation
+  work, and thanks to CEA.fr for sponsoring remaining bugfixes and cleanups!
+
 Release 3.6.0 (2019-11-03)
 --------------------------