Use O_CREATE for bolt.Open(..) only when file doesn't exist

+ By default, etcd-io/bbolt appends an O_CREATE flag to the flags passed in
  no matter which mode the user attempts to open the db ..
    https://github.com/etcd-io/bbolt/blob/v1.3.5/db.go#L197...L213
+ This appears to be bolt's design decision where they let you create a
  file that is only writeable by the process if it doesn't exist - even
  when it is OPEN-ed in READ-ONLY mode.
+ However, using an O_CREATE on a file that's owned by a different user
  but READ-able by all users, seems to be causing a permissions error.
    - While I cannot seem to reproduce this issue on osX, this seems to
      manifest on windows which complains ERROR_ACCESS_DENIED.

+ The proposal here is to overwrite Bolt.Options' OpenFile with a custom
  implementation that appends the O_CREATE file only if the file does
  not exist - sticking to bolt's design decision.
+ Should fix blevesearch#1623

index/scorch/scorch.go

@@ -212,6 +212,15 @@ func (s *Scorch) openBolt() error {
 	var rootBoltOpt = *bolt.DefaultOptions
 	if s.readOnly {
 		rootBoltOpt.ReadOnly = true
+		rootBoltOpt.OpenFile = func(path string, flag int, mode os.FileMode) (*os.File, error) {
+			// Bolt appends an O_CREATE flag regardless.
+			// See - https://github.com/etcd-io/bbolt/blob/v1.3.5/db.go#L210
+			// Use os.O_RDONLY only if path exists (#1623)
+			if _, err := os.Stat(path); os.IsNotExist(err) {
+				return os.OpenFile(path, flag, mode)
+			}
+			return os.OpenFile(path, os.O_RDONLY, mode)
+		}
 	} else {
 		if s.path != "" {
 			err := os.MkdirAll(s.path, 0700)