multithread / multiprocess safe dataset reads

[rlizzo]

Motivation and Context

Why is this change required? What problem does it solve?:

This is the initial work on making hangar reads thread/process safe. The batch_get function is the preffered method for user interaction.

If it fixes an open issue, please link to the issue here:

Addresses some of the requirements to enable: #13

Description

Describe your changes in detail:

DatasetDataReader class no longer contains references to lmdb environments. This change was made as it cannot be serialized and sent across processes.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• 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)

Is this PR ready for review, or a work in progress?

• Ready for review
• Work in progress

How Has This Been Tested?

Put an x in the boxes that apply:

• Current tests cover modifications made
• New tests have been added to the test suite
• Modifications were made to existing tests to support these changes
• Tests may be needed, but they are not included when the PR was proposed
• I don't know. Help!

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have signed (or will sign when prompted) the tensorwork CLA.
• I have added tests to cover my changes.
• All new and existing tests passed.

[rlizzo]

Merging now!

[rlizzo]

Thanks for the review @hhsecond!