If client-side verify_key is not enabled, don't check it automatically

[rlerdorf]

libmemcached's key verification is only done if OPT_VERIFY_KEY is enabled. We should honour this setting in the PHP extension as well and not do it automatically.
This addresses #546

Note that I am still not completely sold on making this change.

[SpencerMalone]

Implementation wise, this LGTM. In terms of:

Note that I am still not completely sold on making this change.

I wanna note that as an impacted user: explicitly documenting previously the breaking change and leaving it in place, or making us default to enabling MEMCACHED_BEHAVIOR_VERIFY_KEY and calling out that change are both 1000% acceptable to me! As long as I have a heads up that there are key requirement changes when I upgrade, I think it's totally acceptable, the only thing that is scary is upgrading and finding a new requirement without knowing what the requirements are or why keys started failing.

[sodabrew]

I was going to make this suggestion, but then I realized the setting is defined as 64-bits. So it goes!

php-memcached/php_memcached.c

Line 4229 in 7fefcb7

REGISTER_MEMC_CLASS_CONST_LONG(OPT_VERIFY_KEY, MEMCACHED_BEHAVIOR_VERIFY_KEY);

Suggested change

	zend_bool s_memc_valid_key_ascii(zend_string *key, uint64_t verify_key)
	zend_bool s_memc_valid_key_ascii(zend_string *key, zend_bool verify_key)

[rlerdorf]

Yes, that is why I made it a uint64_t, but perhaps we should flip that one to a REGISTER_MEMC_CLASS_CONST_BOOL ?