implement buffer pool for message decoding

[noot]

Currently, the library allocates a new buffer for every incoming message (ie. every call to ReadStreamMessage) which may allocate a buffer up to length max(uint32), if that's what's passed by the caller. This is bad because many successive or concurrent reads can cause us to run out of memory resulting in a program crash. Also memory allocation uses a lot of CPU, which may be an issue depending on the application.
The library should only allocate up to a certain amount of buffers, and add them to a pool for re-use. The maximum size/amount of buffers should be configurable by the user of the lib.