Fix potential slow/hanging host resolution when sending mail

[marcelmay]

On MacOS, it can happen that name resolution hangs eg when sending mail:

"main" #1 prio=5 os_prio=31 tid=0x00007fa40f00c800 nid=0x1603 runnable [0x0000700008e3c000]
   java.lang.Thread.State: RUNNABLE
	at java.net.Inet4AddressImpl.getHostByAddr(Native Method)
	at java.net.InetAddress$2.getHostByAddr(InetAddress.java:933)
	at java.net.InetAddress.getHostFromNameService(InetAddress.java:618)
	at java.net.InetAddress.getCanonicalHostName(InetAddress.java:589)
	at javax.mail.internet.InternetAddress.getLocalHostName(InternetAddress.java:655)
	at javax.mail.internet.InternetAddress._getLocalAddress(InternetAddress.java:630)
	at javax.mail.internet.InternetAddress.getLocalAddress(InternetAddress.java:601)
	at javax.mail.internet.UniqueValue.getUniqueMessageIDValue(UniqueValue.java:76)
	at javax.mail.internet.MimeMessage.updateMessageID(MimeMessage.java:2211)
	at javax.mail.internet.MimeMessage.updateHeaders(MimeMessage.java:2242)
	- locked <0x00000007b571b160> (a javax.mail.internet.MimeMessage)
	at javax.mail.internet.MimeMessage.saveChanges(MimeMessage.java:2198)
	at javax.mail.Transport.send(Transport.java:99)
	at com.icegreen.greenmail.util.GreenMailUtil.sendMimeMessage(GreenMailUtil.java:263)

This happens even with host IPv4 and IPv6 entries in /etc/hosts.

This fix avoids the lookup by setting mail.host property to configured mail server. See jakarta.mail.internet.InternetAddress#_getLocalAddress.