Introduce TableLastModifiedMetadataBatch and implement BaseAdapter.calculate_freshness_from_metadata_batch

[MichelleArk]

resolves #138
docs dbt-labs/docs.getdbt.com/#

Problem

No base abstraction / implementation exists for calculating metadata-based freshness in batch

Solution

1. Introduce TableLastModifiedMetadataBatch capability, for adapter implementations to indicate whether a batch version of metadata-based freshness calculation is available
   2.Implement BaseAdapter.calculate_freshness_from_metadata_batch

• groups sources into information schemas and runs 1 query / information schema under the hood, returning a response per query and a mapping of relations -> freshness results
• I also considered updating calculate_freshness_from_metadata but the interfaces were different enough that it would have been difficult to do in a backward-compatible way. However, calculate_freshness_from_metadata was rewritten in terms of calculate_freshness_from_metadata_batch so there are some wins in terms of code reuse here.

3. Added needs_conn to BaseAdapter.execute_macro for callers to signal whether a macro will require an open connection or not. By default, this is false (preserving existing behaviour), but calculate_freshness_from_metadata_batch sets it to true.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR. Tests in supported adapters
  • dbt-snowflake: add Capability.TableLastModifiedMetadataBatch support dbt-snowflake#928
  • dbt-redshift: TableLastModifiedMetadataBatch capability dbt-redshift#744
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

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[mikealfare]

I left a few comments/questions, some might even be useful.