chan_sip: Give more time for TCP/TLS threads to stop.

The unload process currently tells each TCP/TLS to terminate but
does not wait for them to do so. This introduces a race condition
where the container holding the threads may be destroyed before
the threads are able to remove themselves from it. When they
finally do the container is invalid and can't be used causing a
crash.

A previous change existed which waited a bit to wait for any
stranglers to finish. This change extends this and waits longer.

ASTERISK-25961 #close

Change-Id: Idc6262b670ca49ede32061159e323b7b63c6f3c6