fix: Deprecation warning & float to int conversion

[deer-wmde]

With PHP 8.1, I got this output when running a batch:

<br />
<b>Deprecated</b>:  Implicit conversion from float 0.1 to int loses precision in <b>/var/www/html/quickstatements/public_html/quickstatements.php</b> on line <b>1137</b><br />
{"status":"OK","command":{"action":"add","item":"Q2","property":"P1","what":"statement","datavalue":{"type":"wikibase-entityid","value":{"entity-type":"item","id":"Q1"}},"meta":{"message":"","status":"RUN","id":0},"summary":"#temporary_batch_1738321266983","status":"done","message":"Statement already exists as Q2$5A1CDA8E-2620-4665-ADEF-405FAED776A2"},"last_item":""}

Which made the UI fail to parse the response.

Thus I looked at the usage of $this->sleep and found only this one instance, where it seemed safe to just multiply to microseconds and use usleep instead.