Enable heap profiling with jemalloc.

No need for PR!  :-D

The jemalloc library does heap analysis.  For rippled, we need to dump a description of the heap from within the
running application via a function call.  Details on how to perform this
activity in general, as well as how to download the software, are available on
the jemalloc site: [...]

(Quibble: "acquire" means ownership...)