Previously we just assumed that if same settings ID is found in
SETTINGS, it is enough to process last seen entry.  But it turns out
it is not enough for SETTINGS_HEADER_TABLE_SIZE.  If we have 0 and
4096 for SETTINGS_HEADER_TABLE_SIZE in one SETTINGS, we must first
shrink dynamic table to 0 and then enlarge it to 4096.  This means
that we have to remember the minimum value and last value.

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.