decode: several fixes

[yaauie]

This changeset includes three fixes related to decoding:

1. correctly handle escaped newlines and carriage returns in extension values
2. avoid data-loss and corruption by identifying and intercepting invalid payloads
3. avoid data-loss by ensuring a buffered codec flushes its internal buffer

CR/LF in Extension Values

Per CEF spec v25 (2017-09-28):

Multi-line fields can be sent by CEF by encoding the newline character
as \n or \r. Note that multiple lines are only allowed in the value part
of the extensions

While this plugin has long encoded multiline extension values and other
escape sequences, our decode has only supported escaped backslashes or
escaped equals signs, and with this change becomes compliant with this portion
of the spec. Note that due to the preexisting newline-centric normalization,
round-trip encode/decode cycle is only semantically guaranteed.

Intercepting Invalid Payloads

When encountering malformed-CEF or non-CEF payloads, this plugin now emits
helpful descriptive log messages, and prevents data-loss and corruption by
emitting an event tagged with _cefparsefailure containing the bytes it
received.

This set of changes catches 3 distinct cases of malformed payloads.

• missing one or more of the 7 required CEF header fields; a payload that
  does not have all 7 unescaped-pipe-terminated header fields cannot be
  reliably interpreted as CEF (prevents corruption).
• containing something OTHER than a sequence of key=value pairs in the
  extensions space (prevent data-loss; previously when extensions were
  invalid they were silently omitted)
• containing unescaped newlines (prevents corruption; previously data after
  the first newline was injected into the currently-parsed extension field)

In catching these classes of malformed inputs, this changeset also
resolves #99 in which our failure
to detect a malformed input proactively caused an unhelpful NoMethodError
message to be logged before a _cefparsefailure-tagged event was emitted.

Buffer Flushing

When configured with delimiter, this plugin creates an internal buffer.
When that buffer is present, this codec needs to implement CEF#flush so that the buffered bytes can be consumed when this codec is being flushed/closed.

[mashhurs]

LGTM!