Add BufferHelper

These patches provide a unified memory buffer interface, which can be used by three main subsystems:

• Saving ZBuffers
• Accumulation buffers
• Standard SoftBufferAllocMgr

It allows both Ephemeral and Persistent buffers, and a mechanism to control several Parameters:

• Allocator of MemBuffers
• Persistent (pool) size policy
• Memory initialization
• Pool (re)size policy

It aims to be more general than !257 (closed), which becomes obsolete after this.

Also closes #107 (closed)