Respect configured AWS profile when calling boto3

[cstruct]

Use a shared boto3.session.Session between pyathena and manual
boto3 calls. This fixes #120

[owenprough-sift]

How similar is this PR to #56 ?

[cstruct]

Aah, I obviously don't know how to search. This PR is very similar, where it differs is that it shares one session with pyathena and that the bug is fixed for newly introduced usages of boto3. I can close this in favor of the old one if that is preferable and it is updated to cover the new places that boto3 is used.

[w0rks74710n]

#125 leverages this approach and introduces a global variable that holds the boto3_session that can be retrieve at any point in time via get_boto3_session(). Happy to decline that PR in favour of this one. As I mention #125, #56 has been lying around for quite some time (outdated version of the code) and #124 addresses the issue in only one instance of the adapter i.e. clean_up_table.