in_monitor_agent renders Filter plugins
Merge pull request #514 from fluent/filter-support-in-monitor-agent

in_monitor_agent renders Filter plugins
filter out if #filter method returns nil record
rewrite filter_grep using new filter plugin API
Merge pull request #515 from fluent/filter_nil

filter out if #filter method returns nil record
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Merge pull request #517 from fluent/test_driver_routing

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