Make some options dynamically configurable

[yoshinorim]

Some options like rocksdb_bytes_per_sync should be configurable dynamically.

[jkedgar]

Does this mean that you want a global variable in MyRocks so that you can configure this option by using "Set rocksdb_bytes_per_sync = "?

What other options should be this way?

[yoshinorim]

I'd like to start from dynamically configurable DB options. Static options such as wal_dir, rocksdb_data_dir are not targets. For CF/Table options, I think we need more discussions.

[jkedgar]

Is there a list of dynamically configurable options?

[yoshinorim]

^^ @siying @rven1

[siying]

Search options.h and all dynamic configurable options will write in comments "dynamic changeable".

[jkedgar]

The SetOptions() method appears to operate on a column family - the default column family if no column family is specified. Do I need to iterate through all the column families when an option is set via MySQL or do I just set it for the default column family?

The bytes_per_sync setting does not appear to be dynamically configurable. It is not listed as such in options.h and doesn't show up in util/options_helper.cc. Am I missing something?