Terminate WebDrivers, always

[thc202]

Change CrawlTaskConsumer to always close the crawler, in a finally block.
Change WebDriverBackedEmbeddedBrowser to close/quit the browser in a different thread than the one being called to avoid interruptions, which could prevent the WebDriver (e.g. chromedriver, geckodriver) from being terminated, leading to process leaks.
Quit the browser directly instead of using WebDriverManager when it does not contain any as the WebDriver might not have been created through it (e.g. when using custom Provider<EmbeddedBrowser>).

---

Downstream change: zaproxy#20.

---

Sample exception when failed to quit the WebDriver because of the interrupt:

NettyWebSocket initial request interrupted
java.lang.InterruptedException
	at java.base/java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:385)
	at java.base/java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1999)
	at org.openqa.selenium.remote.http.netty.NettyWebSocket.<init>(NettyWebSocket.java:99)
	at org.openqa.selenium.remote.http.netty.NettyWebSocket.lambda$create$3(NettyWebSocket.java:128)
	at org.openqa.selenium.remote.http.netty.NettyClient.openSocket(NettyClient.java:107)
	at org.openqa.selenium.devtools.Connection.<init>(Connection.java:78)
	at org.openqa.selenium.firefox.FirefoxDriver.maybeGetDevTools(FirefoxDriver.java:269)
	at org.openqa.selenium.remote.RemoteWebDriver.quit(RemoteWebDriver.java:438)
	at org.openqa.selenium.firefox.FirefoxDriver.quit(FirefoxDriver.java:323)
…

which prevents both WebDriver and browser to quit.

[amesbah]

LGTM!