Implement streaming WAL segment iterator

Currently, WALSegmentIterator implementations read to the end of
the end of their list of segments and return EOF. This commit adds
the ability to push additional segments to in-process iterators and
notify their callers that new segments are available. This is only
implemented for the file-based iterator but other segment iterators
may get this implementation in the future or have a wrapping
iterator provide a polling-based implementation.

litestream.go

@@ -39,10 +39,12 @@ const (
 
 // Litestream errors.
 var (
-	ErrNoGeneration     = errors.New("no generation available")
-	ErrNoSnapshots      = errors.New("no snapshots available")
-	ErrNoWALSegments    = errors.New("no wal segments available")
-	ErrChecksumMismatch = errors.New("invalid replica, checksum mismatch")
+	ErrDBClosed          = errors.New("database closed")
+	ErrNoGeneration      = errors.New("no generation available")
+	ErrGenerationChanged = errors.New("generation changed")
+	ErrNoSnapshots       = errors.New("no snapshots available")
+	ErrNoWALSegments     = errors.New("no wal segments available")
+	ErrChecksumMismatch  = errors.New("invalid replica, checksum mismatch")
 )
 
 var (