Add digest/sha2 which appears to resolve LOGSTASH-1509
Add recurse method for doing breadth-first traversal of the AST

This will be used by the filter flush compiler
Add generation of a flush lambda for each filter.

This allows filters to flush and have any generated events proceed
downward through the config as you would expect, respecting any
branches or future plugins.
Die on IOError which occurs when reading from a closed STDIN
Make filter_flusher invoke the new (and correct, I hope!) way to flush.

- On shutdown, we will also flush all filters.
- The flusher thread will terminate if we are shutting down.
Clarify the comment
Fix comment generation in the code to avoid newlines.
Add 'max_age' setting to multiline for flushing.

This setting chooses how long (in seconds) an event is considered to be
fresh before it will be automatically flushed.

This is useful for:
* slow log sources to get the 'last event' flushed,
* transaction-id-style events that have no obvious "end" event and also
  are mixed among other-id events in the same stream.

Also:
- Make filters have no teardown by default.
- Remove 'enable_flush' since it is not needed anymore; flush is always
  enabled.
refactor flush
new spool filter and specs, mainly for testing flushing
turn off unrelated test error for now
fix the flush logic, fix the flush compiles code to not include output section
synchronize cross-thread  access to @pending
refactor for performance and readability
synchronize cross-thread access to @spool
unused code
input:udp removed boggus ShutdownSignal handling, morphed loop do into while true, cosmetic reformat
use transcient events and not exceptions for in-flow pipeline signaling
inline flushing into  filterworker
removed now unnecessary flushing thread safety