DBTest: options clean up - part 1

Summary:
DBTest has several functions (Reopen(), TryReopen(), ChangeOptins(), etc
that takes a pointer to options), depending on if it is nullptr, it uses
different options underneath. This makes it really hard to track what
options is used in different test case. We should just kill the default
value and make it being passed into explicitly. It is going to be very
hairy. I will start with simple ones.

Test Plan:
make db_test
stacked diffs, will run test with full stack

Reviewers: sdong, yhchiang, rven, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27687