Makes flat_tree aware of incremental parsing to avoid race conditions with pushes

[anarthal]

Adds the concept of a "temporary working area" to flat_tree. Nodes in this area belong to a partially parsed message, and are hidden from the user. Now flat_tree can be used as the receive response without explicitly handling partial messages.
Changes flat_tree::get_view return type from const vector& to span.
Adds flat_tree::capacity.
Splits generic_flat_response tests to a separate file and adds extra cases.

close #369

[anarthal]

This doesn't change flat_tree to a container (will do as a separate PR) but addresses the race condition issue.

[anarthal]

Merging per Marcelo's request.