Consider changing socketTimeout to milliseconds

[hmueller01]

As @TD-er mentioned in #32 change socketTimeout internal from seconds to milliseconds to avoid the multiplication every time.

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However there is still something odd:

    if (currentMillis - previousMillis >= this->socketTimeout * 1000UL) {
        return false;
    }

Why consider the socketTimeout as seconds here?

It isn't a bug, just odd as you now need to multiply it with 1000 every time you want to check it.

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API would not change by this way:

PubSubClient& PubSubClient::setSocketTimeout(uint16_t timeout) {
    this->socketTimeout = timeout * 1000UL;
    return *this;
}