Add configurable buffering to logging

[ofrobots]

It would be nice to have optional buffering in the the logging client library so that all the higher level layers (e.g. logging-winston and logging-bunyan) and users don't have to build this independently.

Ref: #1977 (comment)

@stephenplusplus Thoughts?
/cc @cristiancavalli

[stephenplusplus]

Since we use gRPC, which keeps an open channel, is batching still beneficial?

[ofrobots]

My understanding, as per the quota policy for Stackdriver Logging, the number of write calls are counted. Whether each of the calls go over the same channel wouldn't affect this, I expect.

@zbjornson has practical experience with the quota.

[stephenplusplus]

This convo started to sound familiar. Let me re-open #1296