Initial support for "mongodb" stream context options

Valid context options are:
    log_insert => function($serverinfo, $newdocument, $insertoptions)
    log_query  => function($serverinfo, $query, $cursoroptions)
    log_update => function($serverinfo, $criteria, $newdoc, $insertoptions, $cursoroptions)
    log_delete => function($serverinfo, $criteria, $insertoptions, $cursoroptions)
    log_getmore => function($serverinfo, $cursorinfo)
    log_killcursor => function($serverinfo, $cursorid)
    log_batchinsert => function($serverinfo, $documents, $flags, $insertoptions)

Note that some things (like $query) may have been modified between the
time the user called the function and the time we emit the callback.
This is because we do rewrite for example the query to support mongos