The wallet's Run method, which is invoked by the syncers to run the background
goroutines necessary to run the mixing client, will be run multiple times by
the RPC syncer (as syncers must be created following loss of the RPC
connection).  However, mixclient does not work properly after the first
context cancelation.  This would cause all mixing activity to silently not
work, technically hanging in mixclient, while never hanging any other wallet
activity.

This can be worked around in wallet without mixclient changes by recreating a
new client each time in wallet's Run method.  Additional synchronization is
required to avoid racing on the mixclient pointer.

Backport of 5609155.

Backport of 2e99259.

Backport of 9f9fd2e.