Offer progress indicator during tax report generation

[LefterisJP]

Problem Definition

During the tax report generation, depending on the timerange, the number of taxable events in that period and the presence of price caches, it may take a long time for the tax report to be complete.

At the moment the only way to kind of watch the progress is to tail the log: tail -f rotkehlchen.log.

This can be improved on and we can provide a progress report to the user.

Task

Provide periodic update to the user via the UI on the progress. This can be achieved through the periodic_query_data() function which is already querying the backend every N seconds.

A specific key can be added to the returned dictionary for when a tax report is underway. As the tax report is generated we can be updating a specific attribute of the Accountant class with the current timestamp and at every periodic query return the last processed timestamp.