fd-tracker: add an fd-tracker util to common

fd-tracker: add an fd-tracker util to common

This commit adds an fd-tracker utility to the common libs.
This interface allows a process to keep track of its open
file descriptors and enforce a limit to the number of file
descriptors that may be simultaneously opened.

The intent is to use this interface as part of the relay daemon
to mitigate file descriptors exhaustion problems that are
encountered when the relay has to handle a large number of streams.

The fd-tracker defines two classes of file descriptors: suspendable
and unsuspendable file descriptors.

Suspendable file descriptors are handles to filesystem objects
(e.g. regular files) that may be closed and re-opened later without
affecting the application.

A suspendable file descriptor can be opened by creating a filesystem
handle (fs_handle) using the fd-tracker. The raw file descritptor
must then be obtained and released using that handle. Closing the
handle will effectively ensure that the file descritptor is closed.

Unsuspendable file descriptors are file descriptors that cannot
be closed without affecting the application's state. For instance,
it is not possible to close and re-open a pipe, a TCP socket, or
an epoll fd without involving some app-specific logic. Thus, the
fd-tracker considers those file descriptors as unsuspendable.

Opening an unsuspendable file descritptor will return a raw file
decriptor to the application. It is its responsability to notify the
fd-tracker of the file descriptor's closing to ensure the number
of active file descriptors can be tracked accurately.

If a request to open a new file descriptors is made to the fd-tracker
and the process has already reached its maximal count of
simultaneously opened file descriptors, an attempt will be made to
suspend a suspendable file descriptor to release a slot.

Suspending a file descriptor involves:
  - verifying that the file is still available on the FS (restorable),
  - sampling its current position,
  - closing the file descriptor.

Note that suspending a file descriptor eliminates the POSIX guarantee
that a file may be unlinked at any time without affecting the
application (provided that it holds an open FD to that
file). Applications using the fd-tracker that need to maintain this
guarantee should open those files as unsuspendable file descriptors.

To protect against unlinking and file replacement scenarios, the
fd-tracker samples the files' inode number when a fs_handle is
created. This inode number will then be checked anytime the handle
is suspended or restored to ensure that the application is made
aware of the file's unavailability. This is preferable to
inadvertently opening another file of the same name if the original
file was unlinked and/or replaced between a fs_handle's suspension
and restoration.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>