Make RandomUint64 block until entropy is available.

This commit modifies the RandomUint64 function so that rather than
returning an io.ErrShortBuffer when the system does not have enough
entropy available, it now blocks until it does have enough.  This means
that RandomUint64 will now always eventually succeed unless the entropy
source is closed (which only really ever happens when the operating system
is shutting down).

The tests have also been updated for the change in semantics to maintain
100% coverage.

Closes #23.

common.go

@@ -508,10 +508,7 @@ func writeVarBytes(w io.Writer, pver uint32, bytes []byte) error {
 // can be properly tested by passing a fake reader in the tests.
 func randomUint64(r io.Reader) (uint64, error) {
 	var b [8]byte
-	n, err := r.Read(b[:])
-	if n != len(b) {
-		return 0, io.ErrShortBuffer
-	}
+	_, err := io.ReadFull(r, b[:])
 	if err != nil {
 		return 0, err
 	}

common_test.go

@@ -691,24 +691,13 @@ func TestRandomUint64(t *testing.T) {
 // and checks the results accordingly.
 func TestRandomUint64Errors(t *testing.T) {
 	// Test short reads.
-	fr := &fakeRandReader{n: 2, err: nil}
+	fr := &fakeRandReader{n: 2, err: io.EOF}
 	nonce, err := btcwire.TstRandomUint64(fr)
-	if err != io.ErrShortBuffer {
+	if err != io.ErrUnexpectedEOF {
 		t.Errorf("TestRandomUint64Fails: Error not expected value of %v [%v]",
 			io.ErrShortBuffer, err)
 	}
 	if nonce != 0 {
 		t.Errorf("TestRandomUint64Fails: nonce is not 0 [%v]", nonce)
 	}
-
-	// Test err with full read.
-	fr = &fakeRandReader{n: 20, err: io.ErrClosedPipe}
-	nonce, err = btcwire.TstRandomUint64(fr)
-	if err != io.ErrClosedPipe {
-		t.Errorf("TestRandomUint64Fails: Error not expected value of %v [%v]",
-			io.ErrClosedPipe, err)
-	}
-	if nonce != 0 {
-		t.Errorf("TestRandomUint64Fails: nonce is not 0 [%v]", nonce)
-	}
 }