CLI uses ballista context instead of datafusion context in local mode

[r4ntix]

Which issue does this PR close?

Closes #219

Rationale for this change

See #219

What changes are included in this PR?

• add concurrent_tasks parameter for ballista local mode, default to all available cores
• use BallistaContext instead of SessionContext
• use BallistaError instead of DataFusionError

Are there any user-facing changes?

add concurrent_tasks parameter for ballista local mode, default to all available cores

[andygrove]

This is looking good. Thanks @r4ntix. I left some feedback.

[andygrove]

This is looking good @r4ntix. Did you need any help with the CI failures?

failures:
    context::tests::test_aggregate_func
    context::tests::test_ballista_show_tables
    context::tests::test_show_tables_not_with_information_schema

[andygrove]

@r4ntix I took the liberty of merging latest from master and fixing merge conflicts

[r4ntix]

@r4ntix I took the liberty of merging latest from master and fixing merge conflicts

Thank you very much for helping to fix the remaining issues, I will keep learning and contributing to ballista :)