build: bump version to 0.15.99

Now that 0.15 branch has been split off, master is 0.15.99 (pre-0.16).

Also clean out release notes.

Tree-SHA512: 160f712fae7bfc51e49e36a27ab01f5c243b79a19a70312df95f9bc5cb8067c70aa88911f741fd1625bee549a70655eaea24d98f6049d98c3c14ee1b3143f4cb

Author: laanwj

configure.ac

@@ -1,7 +1,7 @@
 dnl require autoconf 2.60 (AS_ECHO/AS_ECHO_N)
 AC_PREREQ([2.60])
 define(_CLIENT_VERSION_MAJOR, 0)
-define(_CLIENT_VERSION_MINOR, 14)
+define(_CLIENT_VERSION_MINOR, 15)
 define(_CLIENT_VERSION_REVISION, 99)
 define(_CLIENT_VERSION_BUILD, 0)
 define(_CLIENT_VERSION_IS_RELEASE, false)