rdbms: make LoadBalancer::reallyOpenConnection() handle setting DBO_TRX

rdbms: make LoadBalancer::reallyOpenConnection() handle setting DBO_TRX

Make LoadBalancer::reallyOpenConnection() handle initializing DBO_TRX
instead of Database::__construct().

Also:
* Avoid having the "catch" block appear like it returns a
  half-constructed Database.
* Use the variable name $conn instead of $db to be consistent
  throughout the class. Only send Database::__construct() parameters
  that it recognizes instead of mixing in setLBInfo() data.

Change-Id: Iffc3d1d0713051a164adb51a4c4ee12e4ac887c3