v0.3.0

v0.3.0 includes:

• NewResultMap constructor accepts a int capacity rather than keys array
• Minimize Keys interface
  • remove Identifiers and UniqueIdentifiers methods
  • Keys method now returns a []interface{} after calling the Raw method of
    each key
• Check raw value of Key before adding to key store
• Add LoadNoOp to Strategy interface and implement for all strategies.
  LoadNoOp is called when the dataloader returns the requested result from the
  cache. LoadNoOp should increment the loads counter and not return anything.
• Update Cache interface to include Delete and ClearAll which return a
  boolean. Cache interface should also thread context through the methods.
• Remove unused struct fields
• Remove mutex from key store. Keys are added to the key store in a single
  worker go routine therefore concurrent access under normal use is not
  possible.
• Remove mutex from Counter. Counter will no longer be updated from multiple
  go routines.
• Remove mutex from ResultMap as it will no longer be accessed in multiple go
  routines.
• Remove MergeWith method from ResultMap as it a basic wrapper around a for
  loop.
• Update strategy and dataloader to return callback functions (Thunk and
  ThunkMany). Returning callback functions allow Load/LoadMany to be called
  without blocking but still trigger a worker go routine which can trigger the
  batch function if capacity is hit. Results can then be fetched from after
  calling the returned callbacks.
• Implement Once strategy whose purpose is to wrap around calls to the batch
  function or to trigger a background worker to call the batch function. This
  allows for a consistent api throughout projects using the dataloader if a single
  use dataloader is useful.
• Fix race conditions in strategies.
• add tests.