Replace all tikv-jemallocator to jemallocator

[Xuanwo]

Since tikv/jemallocator has become the upstream of jemallocator, I believe it's time to replace all tikv-jemallocator to jemallocator to avoid confusion.

And we should push a new version to tikv-jemallocator with notes that migrate to jemallocator instead.

[BusyJay]

See #56. I'm afraid we have to keep using tikv-jemallocator.

[Xuanwo]

Hi @gnzlbg, would you like to add either @BusyJay or @tikv/maintainers as a co-owner of the jemallocator-global crate? Thanks in advance.

[Xuanwo]

There is an another option that merge jemallocator-global features into jemallocator (maybe under a feature flag). jemallocator-global is simple and no extra deps. I'm willing to take this issue if it looks good to you.

[BusyJay]

That might work too. However, we have made the claim in the last release that tikv-jemallocator is preferred than jemallocator for new projects. Changing it back and forth may cause more confusions.

On the other hand, even jemallocator is chosen, we will still need to publish two versions till next minor bump, which is unlikely happen until next jemalloc breaking change is released.