Support for Long Living Writers

[nirosys]

We have use cases where users are using ion-c's writers with sockets, in order to stream telemetry data over long periods of time. The current overhead and potential for memory fragmentation, when creating a new writer for each emitted record is too high for these cases.

We need to ensure that a writer can either already support long lifetimes, not relying on position or other values that could overflow, or implement functionality to allow writers to be reset without the overhead of freeing and re-allocating needed buffers.