Add support to allow variable name in path for maintain

[michaelprichardson]

Use the same method in the replicate and delete for maintain count so that a variable can be used in the collection name

{
      collection: 'detail1/$master/details',
      // OR
      collection: 'detail1/$source.fieldValue/details',
},

An added feature would be to allow some operation to the variable before it is used in the collection string