This was OString already; use it that way

Change-Id: Ia2268ce8a1e2111adb609c515e5bfa824afa66ce
FastSerializer: Simplify cache to be fixed sized buffer

This makes writeBytes(), which is called a lot, simpler and thus faster.
E.g. for ~15m calls, this saves ~110m pcycles.

Change-Id: I29d01a1a8651f668aff574e0f015cd2f018eb1cd
Use OUStringBuffers for performance reasons.

Saves 120m pcycles in just 250k calls to ScAddress::Format.

Change-Id: Ibf20cf0843a47cf0fa37a38c4fecf1ae1560412c