Reworking thread termination logic.

Turns out the finish methods weren't called correctly, caused by a
mess up with method names which all sounded too similar and the wrong
one ended up being called. I've reworked this by changing the
thread/writer/reader interfaces, which actually also simplifies them
by getting rid of the requirement for writer backends to call their
parent methods (i.e., less opportunity for errors).

This commit also includes the following (because I noticed the problem
above when working on some of these):

     - The ASCII log writer now includes "#start <timestamp>" and
      "#end <timestamp> lines in the each file. The latter supersedes
      Bernhard's "EOF" patch.

      This required a number of tests updates. The standard canonifier
      removes the timestamps, but some tests compare files directly,
      which doesn't work if they aren't printing out the same
      timestamps (like the comm tests).

     - The above required yet another change to the writer API to
       network_time to methods.

     - Renamed ASCII logger "header" options to "meta".

     - Fixes #763 "Escape # when first character in log file line".

All btests pass for me on Linux FC15. Will try MacOS next.

testing/btest/Baseline/scripts.base.frameworks.logging.ascii-line-like-comment/test.log

@@ -0,0 +1,12 @@
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	test
+#start	2012-07-12-21-00-27
+#fields	data	c
+#types	string	count
+Test1	42
+\x23Kaputt	42
+Test2	42
+#end	2012-07-12-21-00-27