Fixed logger crash on startup when perfect environment variables not set up
PLAT-790: Runner: fixed scalability of memory use

This change implement the code from Runner::Task::runWrapper in a separate
program, enabling the new "runWrapper" to call execv right away. This
enables the wrapper process to release any memory pages inherited from its
parent so that old versions of the modified pages do not need to be kept.
AsyncWriterSource: enable callbacks for fd unregistrations
mongo tmp server in a cc file
Added missing file
Added a note about the requirement
PLAT-783: HttpClient and HttpClientImpl no longer an AsyncEventSource

This change makes HttpClient and HttpClientImpl independent from the
underlying event loop. HttpClientV1 has been adapted to use an internalized
MessageLoop derivative, named "SingletonLoop". The latter has the
particularity of not owning the pointer to its sources, which enables said
sources to registered and unregistered themselves within their implementation
code.

This change modifies the interface to HttpClient in a less than trivial way:
- a new "enableDebug" method has been added to HttpClientImpl, replacing
  AsyncEventSource::debug
- MessageLoop can no longer register HttpClient instances
Merge branch 'master' into http-client-ih

Conflicts:
	service/testing/http_client_test.cc
PLAT-783: adapted HttpClient v2 to new interface