Fix - SocketHandler. Set chunkSize default value

[juan-morales]

Fixes #1570

Sets default value of chunkSize to 65023 (a good reasonable number like a UDP datagram size packet).

At the beggining I tried to set it to PHP_INT_MAX (according the documentation in the php website) but I got the following errors while execution the test testWriteContentWithPlainTextMessage():

stream_set_chunk_size(): The chunk size cannot be larger than 2147483647
 monolog/src/Monolog/Handler/SocketHandler.php:272
 monolog/src/Monolog/Handler/SocketHandler.php:364
 monolog/src/Monolog/Handler/SocketHandler.php:339
 monolog/src/Monolog/Handler/SocketHandler.php:316
 monolog/src/Monolog/Handler/SocketHandler.php:74
 monolog/src/Monolog/Handler/SlackHandler.php:150
 monolog/src/Monolog/Handler/AbstractProcessingHandler.php:48
 monolog/tests/Monolog/Handler/SlackHandlerTest.php:108

I got it with php 7.4

So according the error , the number candidate 2147483647 is the max allowed.

But then I changed my mind and used number 65023, the same used for UDP packets in the UdpHandler in Monolog.

If another number should be use as default, please let me know and I can change the PR asap!

I also fixed/changed some php-doc related issues in the file, and added a simple unit test for this.

Ideas/Suggestions are welcome.

[Seldaek]

Thanks but IMO null is a better default, so it uses the default PHP stream chunk size unless explicitly set by a user.