Link with jemalloc explicitly instead of implicitly via libisc
When jemalloc is linked only implicitly via libisc, it might be pulled to late and the replacement for libc memory allocation routines could come too late which could result in mismatch between which versions of malloc() and free() are being used.
Explicitly link with jemalloc as we do with all other important libraries (libuv, liburcu).
Closes #4404 (closed)
Edited by Ondřej Surý