Multiple-instance support: open multiple instance, one instance update can directly reflect on another instance

[evan0greenup]

Currently, only single instance can be opened.

It would be nice to make notes support multiple instance toward single database be opened simultaneously, because user may edit different pages within a single database.

And the content change in one instance can immediately take effect on another instance.

implementation

avoid data corruption:

• Use transaction feature of underlying database (SQLite).
• Use filelock (OS-level)

Aware the content update when other instance make change:

• save the last update timestamp in a file. If the time is newer than the in-memory record, synchronize the content.
• use file-watcher to keep track of file update.