[PATCH] Make LNT compatible with PostgreSQL

[Yi Kong]

Public LLVM perf server suffers from frequent database locking issues.
SQLite is suspected to cause the problem as it is not designed for
multi-user system like LNT.

The patch makes LNT compatible with PostgreSQL. Although theoretically
MySQL should work as well, it's not tested.

SQLite is still supported and used by default, since setting up
PostgreSQL is difficult and unnecessary if deployed locally.

To use PostgreSQL, you need to create a pgdb manually:
    CREATE DATABASE "lnt.db";
And run:
    lnt create SANDBOX --db-dir postgresql://user@localhost

Cheers,
Yi