Add proper transaction support by using a BigQuery session

[ZimbiX]

Contributes to #11 and #12.

Replaces transaction query buffering with actual transaction support. And introduces Sequel::Model support.

Currently this uses the one session for every query, but the docs say that prevents concurrency. They also say a session gets deleted after 7 days (or 24 hours of inactivity), which I think would mean that limiting the use of the single session to transactions would cause still problems for an app with greater uptime than that. I think I need to make it create a new session at the start of each top-level transaction, and stop using the session at the end of it.

Sequel::Model:

The implementation of #schema_parse_table was incomplete, and so was preventing a Sequel model from being defined (it inspects the table schema). However, it seems that the method isn't actually essential (or at least, isn't anymore?). I don't recall what necessitated implementing it, but removing it does let a Sequel model be defined successfully. For better support, this should probably be added back later with a full implementation.

Note that saving a Sequel model also does work now, but isn't error-free yet:

Sequel::Error: can't express [] as a SQL literal
from /home/brendan/Projects/sequel/lib/sequel/dataset/sql.rb:1319:in `literal_other_append'

#transaction works now, but manually running begin probably doesn't, since it lacks the ; select 1 workaround: googleapis/google-cloud-ruby#9617 (comment).

Todo:

• Fix saving a Sequel model
• Use a new session for each top-level transaction
• Don't use a session outside a transaction
• Update the readme regarding transactions
• Support manually running begin
• Consider other transaction isolation levels