Fix issue where closing a center's connections would kill it

src/Center/Center.ts

@@ -136,7 +136,6 @@ export default class Center extends SmppSession {
 	close(): Promise<void> {
 		return new Promise((resolve, reject) => {
 			this.logger.log1(`Center-${this.id} closing active connections`);
-			this.server.close();
 			this.sessions.forEach((session: any) => {
 				session.close();
 			});
@@ -145,6 +144,10 @@ export default class Center extends SmppSession {
 		});
 	}
 
+	destroy(): void {
+		this.server.close();
+	}
+
 	serialize(): object {
 		return {
 			id: this._id,

src/Client/Client.ts

@@ -73,6 +73,8 @@ export default class Client extends SmppSession {
 		super.defaultMultipleJob = job;
 	}
 
+	destroy(): void {}
+
 	doConnect(): PersistentPromise {
 		this.connectPromise = new PersistentPromise((resolve, reject) => {
 			if (this.status !== this.STATUSES[0]) {

src/SessionManager.ts

@@ -41,6 +41,7 @@ export default abstract class SessionManager {
 		return new Promise<void>((resolve, reject) => {
 			this.logger.log1(`Removing session with id ${session.id}`);
 			session.close();
+			session.destroy();
 			this.sessions = this.sessions.filter(s => s.id !== session.id);
 			resolve();
 		});

src/SmppSession.ts

@@ -125,6 +125,8 @@ export default abstract class SmppSession {
 		});
 	}
 
+	abstract destroy(): void;
+
 	sendSingle(job: Job): Promise<object> {
 		return this.sendPdu(job.pdu);
 	}