open the same db in read only mode

[GopherJ]

Error: Database already open. Cannot acquire lock.

I have one process which does read & write, another process which does read

[GopherJ]

rocksdb allows to open in readonly mode

[lhallam]

See #678

[GopherJ]

rocksdb read only mode cannot sync with the write, secondary db instance doesn't work well either, if we cannot find a very useful and elegant solution, then I think we can abandon this

the whole point is to have 1 R/W + n R instances and the n read instances can sync well with the write