Double the amount of fetched logs with every click on fetch more

[das7pad]

Clicking on There’s more to see - xMiB and fetching 150KiB more than last time is quite inefficient, both in terms of bandwidth usage and in human time spent waiting to click again and again.

This PR is proposing to double the amount of log bytes fetched per click.

E.g. for a 2MiB file and only the tail fetched so far:

• before: 150 -> 300 -> 450 -> ... -> 1800 -> 1950 -> 2000
  -> 15650 fetched (7.8x or 11.7MiB wasted) with 14 requests / 13 clicks
• after: 150 -> 300 -> 600 -> 1200 -> 2000
  -> 4250 fetched (2.2x or 2.3MiB wasted) with 5 requests / 4 clicks

n=0;i=0;v=0;while (n < 2000) { console.log(n); n+=150; v+=Math.min(2000,n); i++ };console.log(i,n,v)
n=0;i=0;v=0;while (n < 2000) { console.log(n); n+=(n||150); v+=Math.min(2000,n); i++ };console.log(i,n,v)

Testing done

Based on #704 for Linux:

stage('2 MiB') {
    node {
        sh "head -c \$(expr 2 \\* 1024 \\* 1024) /dev/urandom | base64 --wrap=1000"
    }
}

(This is generating 2.5MiB of log bytes)

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