Repair Oracle compat version check; dont warn if failed

Repair Oracle compat version check; dont warn if failed

Fixed regression in Oracle dialect that was inadvertently using max
identifier length of 128 characters on Oracle server 12.2 and greater even
though the stated contract for the remainder of the 1.3 series is  that
this value stays at 30 until version SQLAlchemy 1.4.  Also repaired issues
with the retrieval of the "compatibility" version, and removed the warning
emitted when the "v$parameter" view was not accessible as this was  causing
user confusion.

Fixes: #4898
Change-Id: Ieb7b3e093610896c5aa12d0789b63262e0ecf9d8