Options::wal_dir shouldn't end in '/'

Summary: If a client specifies wal_dir with trailing '/', we will fail in deleting obsolete log files. See task #4083746

Test Plan: make check

Reviewers: haobo, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17535
Fix wal_dir sanitizing
Fix default path in geodb_test