perf: read larger chunks in logiterator

[icewind1991]

This speeds up iterating through log files by about 30x in my testing

8192 is chosen because it's the size of the php internal read buffer

[come-nc]

There was 1 failure:

1) OCA\LogReader\Tests\Unit\Log\LogIteratorTest::testGetLines
Failed asserting that actual size 0 matches expected size 2.