libutil: add buffer state management to loggers

currently all loggers can always accept messages and never suggest
flushing buffers. in the future this may change, and at that point
we're already fully set up for it. local loggers should never keep
asynchronous (i.e. network-backed) buffers, disk buffers are fine.
networked loggers will require buffers and periodic flushes later.

Change-Id: Ide2114f5bc17f4a1d289c92ed4f9381a1d59dacf

tests/unit/libmain/progress-bar.cc

@@ -34,7 +34,7 @@ namespace nix
             fmt("downloading '%s'", TEST_URL),
             {"https://github.com/NixOS/nixpkgs/archive/master.tar.gz"}
         );
-        act.progress(TEST_DONE, TEST_EXPECTED);
+        (void) act.progress(TEST_DONE, TEST_EXPECTED);
         auto state = progressBar.state_.lock();
         std::string const renderedStatus = progressBar.getStatus(*state);