[racket] thanks for the db library

Thank you to Ryan Culpepper, et al., for the "db" library.

It's been working rock-solid in my Web app stress-testing so far.

"db/postgresql" performance is good, and having it be pure-Racket 
(rather than calling out to a C library) is very convenient when you 
want multiple Racket threads in the same process talking to the database 
at the same time.

Avoiding extra C libraries in one's process is also good for quality 
assurance.  Memory-related oopses in C code are still commonplace, even 
in prominent open source server software.

Neil V.