[Docs] Fixed single dash in service command examples.

In issue #2948 a crash was reported in processCommand(). Later Oran Agra
(@oranagra) traced the bug (in private chat) in the following sequence
of events:

1. Some maxmemory is set.
2. The slave is the currently active client and is executing PING or
   REPLCONF or whatever a slave can send to its master.
3. freeMemoryIfNeeded() is called since maxmemory is set.
4. flushSlavesOutputBuffers() is called by freeMemoryIfNeeded().
5. During slaves buffers flush, a write error could be encoutered in
   writeToClient() or sendReplyToClient() depending on the version of
   Redis. This will trigger freeClient() against the currently active
   client, so a segmentation fault will likely happen in
   processCommand() immediately after the call to freeMemoryIfNeeded().

There are different possible fixes:

1. Add flags to writeToClient() (recent versions code base) so that
   we can ignore the write errors, and use this flag in
   flushSlavesOutputBuffers(). However this is not simple to do in older
   versions of Redis.
2. Use freeClientAsync() during write errors. This works but changes the
   current behavior of releasing clients ASAP when possible. Normally
   we write to clients during the normal event loop processing, in the
   writable client, where there is no active client, so no care must be
   taken.
3. The fix of this commit: to detect that the current client is no
   longer valid. This fix is a bit "ad-hoc", but works across all the
   versions and has the advantage of not changing the remaining
   behavior. Only alters what happens during this race condition,
   hopefully.

Fix issue #2855

Add notice to install VS2013 Update 5 to make sure it compiles properly.

QForkStartup was called for both the parent and the child cases, but it didn't
have any common code for the two cases. Moved the QForkStartup code to
QForkParentInit and QForkChildRun accordingly and removed the QForkParentInit
call from QForkStartup since it can be called directly by main() leaving
QForkStartup to handle the child case only.

When the QFork architecture and the memory mapped file are not required (i.e.
when the sentinel mode is on or the persistence is off) use dlmalloc instead
of switching to the default runtime allocator.
Using dlmalloc when persistence is off improves significantly the performance
in some cases as described in issue #295.
For the sentinel mode it shouldn't make any significant difference, the reason
to use dlmalloc is for semplicity.

[Setup] Updated the help text for setting the chocolatey apikey.
[Setup] Updated the nuget description.

Replication (including diskless replication) requires persistence to be
available. This fix prevents a crash and returns "ERR unknown command" to
the slave.

Formatting, comments, removed unused counters.

Replaced QForkControl->heapBlockSize since it's a global constant.
Removed the std:: namespace prefix.