Adds buffer_ignore_exceeded_chunk option to Buffer

[dterror-zz]

Controls whether exceeded chunk should raise an exception (and break the chain) or log and ignore
the exception. As discussed in #296

[sonots]

This code will enqueue at lib/fluent/buffer.rb#L207-L209 even if queue size ecceeds buffer_queue_limit. It should not be occured.

[dterror-zz]

@sonots added a test case

[kzk]

-1 for this change. Here're two reasons:

First one is a philosophical thing. Fluentd's importance is 'simplicity'. The more we have options, the more beginners need to learn.

Second, you still have a limit on disk volume anyway. And if we have a disk full error while writing to the buffer, we possibly couldn't recover the data. What we want is an application level handling here.

Is just having a larger value for buffer_queue_limit would satisfy your use case?

[dterror-zz]

@kzk yeah, for individual cases, increasing buffer_queue_limit is the way to go. This motivation for this feature is when a plugin is used in a chain and we want to make sure that failures in this particular plugin won't cause the chain to be broken.
Most of the discussion is in #296 and there's another pr specifically for out_copy #303.