receive_objects can run out of memory when receiving persistent logs

[etremel]

The receive_objects function in Group handles the transfer of both in-memory state and persisted logs for replicated objects. As a result of the way our serialization system works, it receives a single "buffer size" for the size of all serialized state (including current in-memory state and prior values in the persistent log), receives all the data into one buffer, and then hands off to Replicated::receive_object to deserialize it into current state and log entries. However, if the replicated object has a lot of persistent log data (i.e. many prior versions on disk), the size of the receive buffer may be too large to fit into memory.

I already encountered this issue when the stack-allocation of char buffer[buffer_size] failed when attempting to receive 10652222 bytes of log data. A temporary fix was to heap-allocate the buffer instead (with char* buffer = new char[buffer_size]), but eventually we may want to transfer logs to a new node that cannot fit even into heap memory.

One way around this problem would be to create a version of mutils::from_bytes that does not require the entire buffer to be in memory at once, but instead can consume it from a socket, much like mutils::post_object is a version of mutils::to_bytes that can serialize data piecemeal into a socket. (This is why the sender of the huge log does not run out of memory, only the receiver).