Update corpus methods with new spaCy functionality

[bdewilde]

Description

• Use spaCy's new DocBin class for saving/loading Corpus data
• Allow parallel processing when adding texts/records to a Corpus
• Bump min spaCy version accordingly, to 2.2.0

Motivation and Context

Just availing myself of new spaCy functionality, and replacing some hacks in the process.

How Has This Been Tested?

All tests pass, including a couple new ones.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation, and I have updated it accordingly.