MEDIUM: stick-table: Stop handling stick-tables as proxies.

This patch adds the support for the "table" line parsing in "peers" sections
to declare stick-table in such sections. This also prevents the user from having
to declare dummy backends sections with a unique stick-table inside.
Even if still supported, this usage will become deprecated.

To do so, the ->table member of proxy struct which is a stktable struct is replaced
by a pointer to a stktable struct allocated at parsing time in src/cfgparse-listen.c
for the dummy stick-table backends and in src/cfgparse.c for "peers" sections.
This has an impact on the code for stick-table sample converters and on the stickiness
rules parsers which first store the name of the dummy before resolving the rules.
This patch replaces proxy_tbl_by_name() calls by stktable_find_by_name() calls
to lookup for stick-tables stored in "stktable_by_name" ebtree at parsing time.
There is only one remaining place where proxy_tbl_by_name() is used: src/hlua.c.

At several places in the code we relied on the fact that ->size member of stick-table
was equal to zero to consider the stick-table was present by not configured,
this do not make sense anymore as ->table member of struct proxyis fow now on a pointer.
These tests are replaced by a test on ->table value itself.

In "peers" section we do not have to temporary store the name of the section the
stick-table are attached to because this name is obviously already known just after
having entered this "peers" section.

About the CLI stick-table I/O handler, the pointer to proxy struct is replaced by
a pointer to a stktable struct.
diff --git a/include/types/applet.h b/include/types/applet.h
index 4786b31..b7b297c 100644
--- a/include/types/applet.h
+++ b/include/types/applet.h
@@ -144,7 +144,7 @@
 		} errors;
 		struct {
 			void *target;		/* table we want to dump, or NULL for all */
-			struct proxy *proxy;	/* table being currently dumped (first if NULL) */
+			struct stktable *t;	/* table being currently dumped (first if NULL) */
 			struct stksess *entry;	/* last entry we were trying to dump (or first if NULL) */
 			long long value;	/* value to compare against */
 			signed char data_type;	/* type of data to compare, or -1 if none */