MailKit 3.0.0

* Removed APIs marked as [Obsolete] in 2.x. * Simplify Fetch()/FetchAsync() APIs by using a new IFetchRequest parameter instead. Made previous APIs into extension methods to aid in porting from 2.x. * Replaced Add/Remove/SetFlags() APIs with Store()/StoreAsync() and simplified the APIs by using a new IStoreFlagsRequest parameter. Made previous APIs into extension methods to aid in porting from 2.x. * Replaced Add/Remove/SetLabels() APIs with Store()/StoreAsync() and simplified the APIs by using a new IStoreLabelsRequest parameter. Made previous APIs into extension methods to aid in porting from 2.x. * Simplify Append()/AppendAsync() APIs by using a new IAppendRequest parameter instead. Made previous APIs into extension methods to aid in porting from 2.x. * Simplify Replace()/ReplaceAsync() APIs by using a new IReplaceRequest parameterinstead. Made previous APIs into extension methods to aid in porting from 2.x. * Updated SmtpClient.Send()/SendAsync() methods to return a string. (issue #1161) * Added support for the SCRAM-SHA*-PLUS SASL mechanisms. (issue #950) * Added authzid support for SCRAM SASL mechanisms. * Added support for the ANONYMOUS SASL mechanism. * Added support for an HttpsProxyClient. (issue #1251) * Added AcceptedKeywords and PermanentKeywords to IMailFolder. (issue #1256) * Rewrote NTLM support based on official specs. Now supports channel-binding and using the default system credentials. * Modified ImapFolder.Fetch(int, int, ...) to shortcut if ImapFolder.Count == 0. * Updated SmtpClient to append an ORCPT arg to RCPT TO commands and to hex-encode the ENVID parameter value. * Improved/simplified logic for ranking SASL authentication mechisms for each client. * Added SaslMechanism.ChallengeAsync() to facilitate future SASL mechanisms that may need to make network requests such as Kerberos/GSSAPI and perhaps even future/custom OAuth2 implementations. * Always set SearchResults.Count/Min/Max properties if we can. * Throw TimeoutException is case of a network time out. (issue #1269) * Fixed parsing of IMAP flag lists to handle lowercase flag names. (issue #1277) * Use OrdinalIgnoreCase when comparing "EARLIER" atom token. * Avoid unnecessary string copies. (issue #1292) * Drop support for .NET 4.5 and replace it with .NET 4.5.2 * Simplified event emissions based on EXISTS and EXPUNGED notifications. A CountChanged event is now *always* emitted when the server sends an EXISTS notification.