Upgrade jemalloc to v0.3

[brson]

Edit: A new version of jemallocator has been released. The task here is to upgrade jemalloc, jemalloc-sys, and jemalloc-ctl.

Should wait until after #4416 (comment) lands.

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I've only just discovered that jemallocator/master is at version 0.3.0, but the latest jemallocator crate (the one we are linking to) is only at 0.1.9 and was published last July.

When I did the toolchain upgrade that refactored our jemalloc code I moved from @BusyJay's fork (I think) to the published version. I could have reverted jemallocator progress by doing so.

So maybe the published versions of jemallocator are deprecated. Not sure. I've asked here.

We may need to change the dependency back to a git revision.

I marked this high priority because I think it is a release blocker to be on the "wrong" version of our allocator.

[brson]

jemallocator v0.3 is released

[brson]

jemalloc-ctl, which we depend on, still hasn't been updated for 0.3. I've been testing the pre-release version and will push to get it published.