lfs/batcher: un-buffer the input channel

In preparation for the next commit where I'll add a Truncate() method to the
batcher, the `input` channel needs to guarentee to its callers that all items
within it have been processed (specifically before returning from `Add()`).

Since the input channel was buffered, the following situation could easily occur:
1. Several calls to Add() are issued, which get buffered by the "input" channel.
2. Items are processed on a separate goroutine, draining the buffer.
3. While the buffer still has items in it, a call to Truncate() causes an
   alternate select/case branch to fire (leaving all of the buffered items to be
   dropped on the floor).
4. A batch is returned containing all of the _processed_ items from `input`.
5. Buffered items are (perhaps) returned in the next batch.

Since the time spent blocking is short and I could not find instances where
this channel buffering is relied upon, this made the most sense of all the
synchronization methods (`WaitGroup`s, `Cond`s, etc)

lfs/batcher.go

@@ -19,7 +19,7 @@ type Batcher struct {
 func NewBatcher(batchSize int) *Batcher {
 	b := &Batcher{
 		batchSize:  batchSize,
-		input:      make(chan interface{}, batchSize),
+		input:      make(chan interface{}),
 		batchReady: make(chan []interface{}),
 	}
 
@@ -31,7 +31,7 @@ func NewBatcher(batchSize int) *Batcher {
 // goroutines.
 func (b *Batcher) Add(t interface{}) {
 	if atomic.CompareAndSwapUint32(&b.exited, 1, 0) {
-		b.input = make(chan interface{}, b.batchSize)
+		b.input = make(chan interface{})
 		go b.acceptInput()
 	}