[pg] Add Connection abstraction

Only the methods used in the "submit" function from the following
projects were declared:

 * pg-copy-streams
 * pg-query-stream
 * pg-cursor

types/pg/index.d.ts

@@ -17,6 +17,7 @@ export interface ConnectionConfig {
     host?: string;
     connectionString?: string;
     keepAlive?: boolean;
+    stream?: stream.Duplex;
 }
 
 export interface Defaults extends ConnectionConfig {
@@ -89,6 +90,48 @@ export interface ResultBuilder extends QueryResult {
     addRow(row: any): void;
 }
 
+export interface QueryParse {
+    name: string;
+    text: string;
+    types: string[];
+}
+
+export interface BindConfig {
+    portal?: string;
+    statement?: string;
+    binary?: string;
+    values?: Array<(Buffer | null | undefined | string)>;
+}
+
+export interface ExecuteConfig {
+    portal?: string;
+    rows?: string;
+}
+
+export interface MessageConfig {
+    type: string;
+    name?: string;
+}
+
+export class Connection extends events.EventEmitter {
+    readonly stream: stream.Duplex;
+
+    constructor(config?: ConnectionConfig);
+
+    bind(config: BindConfig | null, more: boolean): void;
+    execute(config: ExecuteConfig | null, more: boolean): void;
+    parse(query: QueryParse, more: boolean): void;
+
+    query(text: string): void;
+
+    describe(msg: MessageConfig, more: boolean): void;
+    close(msg: MessageConfig, more: boolean): void;
+
+    flush(): void;
+    sync(): void;
+    end(): void;
+}
+
 export class Pool extends events.EventEmitter {
     // `new Pool('pg://user@localhost/mydb')` is not allowed.
     // But it passes type check because of issue: