In case we want to send a lot of data,
and the receiver is slower than the sender.
This will first fill up the receivers queues and after this
eventually also the senders queues,
until the socket is temporarily unable to accept more data to send.
select_write is done with an timeout of zero,
which makes the select call used always return immediately:
(see http://man7.org/linux/man-pages/man2/select.2.html)
This means that every marginal unavailability will make it return false
for is_writable and therefore httplib will immediately abort the transfer.
Therefore make this values configurable in the same way
as the read timeout already is.
Set the default write timeout to 5 seconds,
the same default value used for the read timeout.
According to RFC 3493 the socket option IPV6_V6ONLY
should be off by default, see
https://tools.ietf.org/html/rfc3493#page-22 (chapter 5.3).
However this does not seem to be the case on all systems.
For instance on any Windows OS, the option is on by default.
Therefore clear this option in order to allow
an server socket which can support IPv6 and IPv4 at the same time.
I use a custom TaskQueue, with variable number of workers, adding workers on demand is an easy task when new connection arrive (in enqueue function) however i need another funtion to be called even (or better) went no new connections arrives to reduce workers count. I only added a new virtual method in TaskQueue class to allow custom class to adjust workers size over time. Even if this methods is called frequenlty custom class can keep a "last_update" counter to check if need to adjust worker count or any other internal task. Without this function i need an external thread to make this adjust task.
* Fixed error:
ULONG_MAX is defined in the limits.h header file. Putting #include <climits>
```
httplib.h: In function ‘bool httplib::detail::read_content_chunked(httplib::Stream&, httplib::ContentReceiver)’:
httplib.h:1921:22: error: ‘ULONG_MAX’ was not declared in this scope
if (chunk_len == ULONG_MAX) { return false; }
^~~~~~~~~
httplib.h:1921:22: note: suggested alternative: ‘_SC_ULONG_MAX’
if (chunk_len == ULONG_MAX) { return false; }
^~~~~~~~~
_SC_ULONG_MAX
```
* Move #include <climits> to after #include <cassert>
