Added support for accessing cached offsets.

[Fischiii]

Added the possibility to access the local cached offsets of a consumer using rd_kafka_get_watermark_offsets as always fetching the offsets was to slow for our use case.

If there is already a way to access the currently cached offsets ignore this PR and please point me in the right direction.

[kenneth-jia]

Hi, @Fischiii
Thank you for pointing it out!
Instead of creating new interfaces (e.g. beginningOffsetsCached/endOffsetsCached), maybe we could reuse the existed ones.
I'll refine the beginningOffsets(..., timeout)/endOffsets(..., timeout), to make them return cached offsets if timeout is 0.
(but please wait a few more days for the PR, -- due to my access limit)

[Fischiii]

@kenneth-jia I applied your suggested changes. I think it is a better solution than what I first proposed.