Replace pinfo->layer_names as a string with pinfo->layers as a wmem_list of

Replace pinfo->layer_names as a string with pinfo->layers as a wmem_list of
protocol IDs. This is substantially more efficient, which means we can build it
all the time rather than only if tree (in my benchmarks the extra time taken is
not large enough to be statistically significant even over tens of thousands of
packets).

This fixes what was probably a bug in btobex that relied on layer_names for
non-tree dissection. It also enables a much simpler fix for
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=9303

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@53089 f5534014-38df-0310-8fa8-9805f1628bb7