optionally read rocksdb configfile

[fm3]

new command line option -r <filepath> causes a rocksdb options file to be read.

The two options create_if_missing and create_missing_column_families from the file are ignored and are always set to true.

We still use (and create) the column families specified by the command line parameter -c (rather than constructing them from the config file. We will change that in a future PR)

A sample rocks-config.ini looks like this

[Version]
  rocksdb_version=5.11.3
  options_file_version=1.1

[DBOptions]
  stats_dump_period_sec=700

[CFOptions "default"]

fixes #18

[jstriebel]

LGTM. But it would be great if we could test this, not sure if that's maybe to complicated, though.

[normanrz]

What is the reason for keeping -c?

[fm3]

@normanrz I did not want to change the default behavior in this PR, for it was created to support global options. Dropping -c would make the config file a hard requirement. We can do that if you think it would be a real benefit, but I would defer it to a separate PR

[normanrz]

@fm3 Ok. Let's defer. But I think it would be great to drop the custom -c parameter and rely on the standard RocksDB options (not only for global options, but also for column families including their options)

[fm3]

@jstriebel I added some tests

[jstriebel]

👍