* Makes flat_tree implementation use a custom buffer. This allows:
* Never dangling nodes (previously, node values could dangle after calling reserve() or if notify_done() wasn't called).
* Reduced memory consumption
* Increased runtime speed
* Changes flat_tree assignment to the usual signature and semantics
* Fixes a bug causing an assertion to trigger when copy-constructing an empty flat_tree.
* Changes basic_node operator== and operator!= return type
* Adds generic_flat_response, basic_tree, tree, view_tree, flat_tree to the reference page.
* Adds a missing resp3:: qualifier to all names in the reference page that belong to the resp3 namespace.
* Adds reference documentation to flat_tree.
* Mentions generic_flat_response in the discussion.
* Adds operator!= for basic_node to basic_node's reference page.
* Adds test_flat_tree.
close#357close#354close#352
* Changes cancel_on_connection_lost default to false
* Deprecates cancel_on_connection_lost and cancel_if_not_connected
* Fixes a TODO in test_conn_exec_cancel
close#323
* Adds support for asio::cancel_after in connection::{async_exec, async_run}
* Adds cancel_after tests
* Adds an example on using asio::cancel_after
* Adds a discussion page on timeouts and the `cancel_if_unresponded` flag
close#226
