Allow re-using the ELF/DWARF read_context when loading a corpus group

Allow re-using the ELF/DWARF read_context when loading a corpus group

Right now, when loading each corpus of a group, a new read_context is
created and destroyed.  That makes thousands of corpora that are
created and destroyed.  Profiling seems to argue that we'd gain in
performance by re-using the first read_context that was created
instead, and re-set it before loading a new corpus.

This is what this patch does.

	* include/abg-dwarf-reader.h (reset_read_context): Declare new
	function.
	* src/abg-dwarf-reader.cc (read_context::elf_paths_): Make this to
	be non const.
	(read_context::initialize): New function to initialize all data
	members.
	(read_context::read_context): Use the new read_context::initialize
	function, rather than initializing data members 'inline' here.
	(reset_read_context): Define a new function to reset a
	read_context so that it can be re-used to load a new corpus.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>