Remove VARCHAR and CHAR precision when loading from temporary table

[chuwy]

Loader loads data in two steps:

1. Loads data from S3 into temporary single-OBJECT-column table
2. Loads data from temporary table into atomic.events

During second step, it also performs extraction and casting to get the right property (e.g. enriched_data:event_version::VARCHAR(128)). But precision in unnecessary in this step, it adds another point of failure in a case where user mutated table manually.

Leaving aside a point that we should avoid mutating table manually, sometimes it can be necessary and in future version we're planning to provide a fine-grained approach with processing manifest.