Decrease memory allocation rate

[Jubilee101]

Valgrind reports our base backup mallocs too many bytes. We need to go over base backup implementation and reuse some buffer, to decrease the number of bytes allocated.

[Jubilee101]

Heaptrack reports that most allocations come from untar, cjson parsing and extracting messages from the copy stream. We'll try to decrease the memory allocated for libarchive and cjson, and reuse one message struct for extraction. We will also try to use the stream buffer directly to hold the payload.