Read only support for rocksdb

[eugeneware]

This PR adds read only rocksdb support.

This PR closes #72 and quite a few comments on #13

There's a new readOnly boolean option to the database options object that can be used to open a rocksdb database in readonly mode.

I've also included some tests which use chmod to set the database to read only to set the various edge cases.

I've also published a version to @eugeneware/rocksdb in case this doesn't get merged for a while for people to use.

[eugeneware]

OK. So looks like I'll need a windows machine to try to debug the appveyer messages.

[vweevers]

It's probably due to:

Caveats: on Windows only the write permission can be changed, and the distinction among the permissions of group, owner or others is not implemented.
https://nodejs.org/api/fs.html#fs_fs_chmod_path_mode_callback

But I don't think we have to use chmod for the tests at all. Just test a db.get() and db.put() on a db with readOnly: true. No need to test other operations like del or batch, or other cases that are handled by RocksDB (i.e. not our code).

[eugeneware]

@vweevers So, just test that it can still work on regular databases, don't worry about testing whether it can work with read-only databases? I can do that :-)

My own use case was using the database from a read-only file system, but you're right, the other major use case is just loading a database without mutating any state, which is probably what most people are trying to do.

[vweevers]

Side note: because readOnly: true implies no lockfile can be written.. does that mean you can open a db from multiple processes? Cool! 😃

[eugeneware]

@vweevers I believe so! I haven't tested this my self with a live writing process. But would definitely work for 2 read only processes.

[vweevers]

I still prefer not using chmod, feels out of scope, but we can postpone that discussion.

[vweevers]

We can simplify some of the tests when upgrading abstract-leveldown.

[vweevers]

3.1.0