Don't create a chart until its Data File is generated

[federico-razzoli]

DataFiles should have a lock. When the Data File is initially created, it should be locked by the thread that created it, until it's populated.

This will solve the problem that Query-Viz starts to attempt generating the charts immediately, but initially charts are not populated.

This will also be a beginning of locking implementation.

[federico-razzoli]

I created a DataFileSet.has_started() method, but it's not used yet.