rdbms: reorganize Database fields and constants

rdbms: reorganize Database fields and constants

Move the constants to the bottom and make more of them private.
Place the configuration fields at the top of the list.
Also, move some related fields closer to each other.

In addition:
* Rename the named lock tracking variable to start with the
  prefix "session" for consistency.
* Remove unused $preparedArgs field.
* Rename $sessionVars and $rttEstimate fields.
* Use short field documentation syntax.
* Make transaction callback fields private.

Change-Id: I7d78be6744723f4d7bb32a75154564ee04eca0f6