identification

This fixes another possible race condition if the same file is looked
up through 2 different paths.

Remove FindChildByOpaqueID(), since it is still susceptible to race
conditions: two lookup calls might be racing, and both may see
FindChildByOpaqueID == nil, creating two Node objects

Instead, NewInode() automatically discards a new Node if it finds one
with existing opaqueID.

The FileID struct has (uint64,uint64) for Dev, Ino. While Dev
typically is uint32 (the FUSE protocol has uint32), it is uint64
analogous to syscall.Stat_t. The resulting 128 bits of ID space is
also ample space for storing hashes of other IDs (eg strings)