sqlalchemy adapter trx begin allow transaction_mode

[shanecarey17]

Allows transaction mode to be set on SAConnection.begin() method calls

[codecov]

Codecov Report

Merging #498 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #498      +/-   ##
==========================================
+ Coverage   93.59%   93.67%   +0.08%     
==========================================
  Files          25       25              
  Lines        3684     3733      +49     
  Branches      195      198       +3     
==========================================
+ Hits         3448     3497      +49     
  Misses        198      198              
  Partials       38       38

Impacted Files	Coverage Δ
aiopg/sa/connection.py	85.14% <100%> (+0.61%)	⬆️
tests/test_sa_transaction.py	86.64% <100%> (+2.24%)	⬆️

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[vir-mir]

LGTM - I'll salt in a couple of days.