adap-435 Use explicit begin transaction

[shreyadatar]

resolves #556
docs dbt-labs/docs.getdbt.com/#

Problem

See #556, Mixing Snowflake begin/end scripting blocks with transactions throws a syntax error.

Solution

By using the more explicit begin transaction instead of begin, the above confusion and syntax error is avoided, therefore allowing both begin/end blocks as well as transactions being used together.

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
• 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

[cla-bot]

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[shreyadatar]

Going to start over to make cla-bot like this.