Revert "[Fuchsia] Disable use of SSH connection multiplexing."

This reverts commit 3da02f9.

Reason for revert: This change improved things, but did not fully resolve the issue, and broke reverse port-forwarding e.g. for net_unittests.

Original change's description:
> [Fuchsia] Disable use of SSH connection multiplexing.
> 
> Connection multiplexing results in unexpected hangs or resets when used
> by our runner scripts, so disable it until the root cause is found and
> resolved upstream.
> 
> Bug: 900790
> Change-Id: I5025a5d652cdcbfe0c206fc4bf42366efbee07be
> Reviewed-on: https://chromium-review.googlesource.com/c/1359489
> Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
> Commit-Queue: Wez <wez@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#613377}

TBR=wez@chromium.org,sergeyu@chromium.org

Change-Id: Ieeebbadcfcf4509bd8a6af0fc6bd914d02ca5c7a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 900790
Reviewed-on: https://chromium-review.googlesource.com/c/1361649
Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613612}

build/fuchsia/boot_data.py

@@ -12,8 +12,6 @@
 import time
 import uuid
 
-# TODO(https://crbug.com/900790): Re-enable multiplexing once upstream issues
-# are resolved.
 _SSH_CONFIG_TEMPLATE = """
 Host *
   CheckHostIP no
@@ -26,8 +24,8 @@
   IdentityFile {identity}
   ServerAliveInterval 2
   ServerAliveCountMax 5
-  ControlMaster no
-  ControlPersist no
+  ControlMaster auto
+  ControlPersist 1m
   ControlPath /tmp/ssh-%r@%h:%p
   ConnectTimeout 5
   """