Akavache file locky me no wanty. Might fix #126

Akavache is deprecating the file system provider in favour of the Sqlite implementation. Probably for good reason. However this would be a breaking change to mmbot if we switched.

I suspect there are very few concurrent Sets going on but this change will off-load Set operations onto a new thread if necessary and queues those saves behind each other. Drastic but if we want high performing key-value store then Redis, Azure or a custom brain implementation is the way to go.

Thoughts? (There may have been beer involved in this code)