TS-2176: do not reset value of api_skip_cache_lookup when reading it

When is_request_cache_lookupable reads the value of api_skip_cache_lookup
it also resets the value to false.  Since the function is called
from HandleRequest, the flag set by the API call will be ignored
by DecideCacheLookup.
Use proper sentences in CHANGES
TS-2195: Deprecate experimental TSHttpTxnCacheLookupSkip API

TSHttpTxnCacheLookupSkip() is in experimental, but can be implemented
by stable API; TSHttpTxnConfigIntSet(txn, TS_CONFIG_HTTP_CACHE_HTTP, 0).