use newer, thread-safe BasicCookieStore

In production logs, we would frequently get
ConcurrentModificationException in CBLHttpClientFactory.addCookies.
Tracing the code for a potential concurrent modification was fruitless.

Updating to a more recent verion of Apache's BasicCookieStore that is
newer than what Android provides and has been rewritten with better
concurrency support stopped the issue from occurring.

(fixes couchbase/couchbase-lite-android-core#4)
Merge pull request #15 from mjq/cookiestore

use newer, thread-safe BasicCookieStore