fslock is dead, long live mutex

[Tim Penhey]

Hi folks,

We have finally managed to exorcise the fslock from the codebase. Both 
1.25 and master no longer refer to it at all. We need to remove it from 
the juju/utils package to make sure that people don't accidentally try 
and use it again.

There is a new replacement, the juju/mutex package. To acquire a mutex, 
you create a mutex.Spec structure. You must at least provide a name, 
delay and clock. The delay is how long the code waits between tries to 
acquire the mutex. Named mutexs are shared between processes on the same 
machine. If a process holds the mutex and the process dies, the mutex is 
automatically released.  A spec can also have a timeout value, and/or an 
abort channel.

On linux this is implemented with abstract domain sockets, on windows it 
uses a named semaphore, and on other platforms a flock is used on a 
named temp file.

https://godoc.org/github.com/juju/mutex

Cheers,
Tim