jemalloc is being officially archived

See

https://github.com/jemalloc/jemalloc

and

https://news.ycombinator.com/item?id=44264958

for discussions.

May need to check if jemalloc is going to be maintenanced in the future or not.

Start by reviewing the jemalloc repository and the linked Hacker News discussion, then locate FoundationDB's jemalloc dependency and build references. Determine whether jemalloc remains maintained and document a concrete maintenance, replacement, or removal path; the issue currently names no files or tests.

Written by the indexing model from the issue text.