Summary:
[Folly] Cut `DestructorGuard` default ctor. It does not actually need one, but its presence can hurt.

```lang=c++
void callback() {
  // compiles, which can hurt:
  DestructorGuard(delayed);

  // intended:
  DestructorGuard guard(delayed);
}
```

Reviewed By: lbrandy

Differential Revision: D10413329

fbshipit-source-id: f84eb0de0290cb09f7ac358044c2ef2d71dd8cea

Summary: folly/experimental/pushmi isn't ready to build in the OSS environment yet, so exclude it.

Reviewed By: Orvid

Differential Revision: D10428301

fbshipit-source-id: d6f033894cc060b560697f6bfa1d1f78102568d3

Summary:
We need this in wormhole, as the capacity of the IOBuf may be very large, but,
the data length is much smaller. Current implementation of coalesce() allocates
large amount of memory unnecessarily.
Adding methods to be able to set headroom and tailroom during coalesce.

Reviewed By: yfeldblum

Differential Revision: D10386802

fbshipit-source-id: a0d787f41ed195eef409b942aaad583fc22692a3

Summary:
Currently `FOLLY_SDT`'s Semaphore is just an global variable at where it was defined. Since it's initialized to 0 it will be put into the `.bss` section. It works fine currently, especially for Executable binaries, because tools, in particular BCC, will be writing to the virtual address directly.

However, recent patches to Linux Kernel supports handling Semaphore (reference counter) natively along with the uprobe event, and it requires the Semaphore address to be allocated in the binary file so Kernel can find correct VMA for the address, especially for Shared Object binaries.

This Diff makes `FOLLY_SDT_DEFINE_SEMAPHORE` to put the Semaphore into a `PROGBITS` section `.probes` that is writeable and allocated. This is a practice compatible with SystemTap's USDT implementation, where the Semaphores also reside in the `.probes` section, as can be found in CPython, libc, MySQL, PostgresSQL, and many applications already.

Reviewed By: yfeldblum

Differential Revision: D10319732

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