servers: Remove synchronization for HeartbeatThread#shutdown().

The synchronization is a leftover from a previous stage of development.
In recent commits synchronization has been removed from the other
methods and can be removed from shutdown too.
Synchronization is not necessary since shutdown is idempotent.
servers: multihoming: Add the matching network in CIDR notation for local addresses to the address mappings.
servers: multihoming: Use POSIX signal USR2 on OSDs to renew their address mappings.
servers: multihoming: Remove org.apache.commons.net dependency.
servers: multihoming: Try finding the localHostAddress and set it as globally accessible.
Prefer local networks over global ones when resolving an UUID.
servers: multihoming: Wait on a monitor in HeartbeatThread to enable immediate address renewals.
servers: multihoming: Move the address renewal signal handler to the HeartbeatThread and make it configurable.

This makes the HeartbeatThread responsible for the signal handling. OSD
and MRC initialize it in regard to the new setting 'renewal_signal'.
The new setting is briefly documented.
servers: multihoming: Add config paramaters for multihoming and address renewal signals.
servers: multihoming: Adapt endpoint registering to the introduced config parameters.

With this commit there are different possibilities to determine the endpoints
that will be propagated at the DIR.

1. multihomed.enabled is set to true and hostname is empty:
   Every valid address will be used with the corresponding network.
   It will be attempted to find the local host automatically and then use it as
   the default.

2. multihomed.enabled is set to true and hostname is set:
   As in the first case, but the configured hostname will be used to determine
   the default.

3. multihomes.enabled is not set and neither hostname nor listen.address
   It will be attempted to find the first non-local address and use it as a
   single endpoint matching any network. If this isn't successful it will be
   attempted to use the first local address. If this fails, too, the localhost
   will be used.

4. multihome.enabled is not set but hostname or listen.address
   The configured hostname or if not set listen.address will be used to as a
   single endpoint matching any network.
servers: multihoming: Adapt to comments in the review for revision  7c4084507005
servers: multihoming: Refactor NetUtils.getReachableEndpoints() and move more logic to the HeartbeatThread.
servers: multihoming: Improve renewal of address mappings.
servers: multihoming: Refactor HearbeatThread and NetUtils to simplify
the overall structure.
servers: multihoming: Restore Java JDK 6 compatibility by replacing a
call to InetAddress.getLoopbackAddress().
docs: multihoming: Improve documentation in response to revision fda1c2415841
docs: Ignore intermediate files created while building LaTeX files.
servers: multihoming: Adapt multihoming to comments in the review for
revision 831bf6495532
servers: multihoming: Renew the UUIDResolver's network list when the
address mapping is changed.