Optimize RefAdvertiser for wire protocol

Optimize RefAdvertiser for wire protocol

The native wire protocol sends ref advertisements in the pkt-line
format, which requires encoding the ObjectId and ref name onto a byte
sequence.  Busy servers show this is a very high source of garbage,
which pushes the garbage collector harder when there are many refs in
the repository (e.g.  70k, in a Gerrit managed repository).

Optimize the side band advertiser by retaining the CharsetEncoder,
minimizing the amount of temporary garbage built during encoding.

Change-Id: I406c654bf82c1eb94b38862da2425e98396134cb