Jemalloc will be used if the cmake option `USE_JEMALLOC` is enabled(which is the
default). To avoid trouble with clang's ASAN, it is disabled by default if the
`SANITIZE` option is enabled.
Since jemalloc has thread cache for small objects, it fills the gap created by
removing klib memory pools.
The `xstrdup` funciton(memory.c) had to be reimplemented on top of `xmalloc` to
make it work with a custom allocator.
