To ensure that cached time buffer is not altered while referencing it,
we rely on inerlocking in std::shared_ptr to acheive this.

This change rewrites logging system of nghttpx.  Previously access log
and error log are written to stderr or syslog and there was no option
to change stderr to something else.  With this change, file path of
access log and error log can be configured separately and logging to
regular file is now added.  To support rotating log, if SIGUSR1 signal
is received by nghttpx, it closes the current log files and reopen it
with the same name.  The format of access log is changed and has same
look of apache's.  But not all columns are not supported yet.

This option is testing the client behavior when it gets response
before sending all request.

Since we do not call on_data_chunk_recv_callback for ignored DATA
chunk, if nghttp2_option_set_no_auto_connection_window_update is used,
application may not have a chance to send connection WINDOW_UPDATE.
To fix this, we accumulate those received bytes, and if it exceeds
certain number, we automatically send connection-level WINDOW_UPDATE.

This commits changes the upper bound of one header field size (the sum
of the length of name and value) to 64KiB by default.  We may add an
option to change this upper bound in the future.