Dynamic backend: large machine type being drained may replace buffer machine of a different type

[sanderegg]

sim4life.io now has X buffer machines of EC2 type Y:

• when a buffer machine is used, it is replaced by a new machine of type Y

If someone uses another type of machine, the user will wait for that machine to be up/running because the buffer only creates machines of type Y.

BUT, once the user finishes using that machine, then the larger machine is:

• drained
• might replace one of the buffer machines,
  --> buffer machine types is then not what is expected (machine type Y + another type), eventually completely replacing the buffer of machine Y type.