borg2: allocate, copy / move, free less memory

From a recent profiling:

The one large item that isn't crypto or I/O is memory management

memmove 17.1% + madvise 6.8% = ~24% of the critical path — copying each ~2 MB chunk between chunker, compressor and encrypter, then handing multi-MB buffers back to the OS.

Start with the profiling results in the issue and trace how roughly 2 MB chunks move between the chunker, compressor, and encrypter, including when multi-MB buffers are returned to the OS. Done means reducing the reported memmove and madvise share of the critical path without changing backup behavior.

Written by the indexing model from the issue text.