Upstream: fix $upstream_cache_last_modified variable.

Due to the u->headers_in.last_modified_time not being correctly initialized,
this variable was evaluated to "Thu, 01 Jan 1970 00:00:00 GMT" for responses
cached without the "Last-Modified" header which resulted in subsequent proxy
requests being sent with "If-Modified-Since: Thu, 01 Jan 1970 00:00:00 GMT"
header.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>
Core: fixed buffer overrun when hash max_size reached.
Fixed possible buffer overrun in "too long header line" logging.

Additionally, ellipsis now always added to make it clear that
the header logged is incomplete.

Reported by Daniil Bondarev.