INT-3453 NIO Always Release Assembler Thread

Fixed hung assembler described in INT-3453.

Used CountDownLatch instead of sleep()

INT-3453 Add Test Case

Currently failing for me - assembler thread stuck on CDL.

Remove redundant `if (TcpNioConnection.this.writingLatch != null)` from `ChannelInputStream#write`

Conflicts:
	spring-integration-ip/src/test/java/org/springframework/integration/ip/tcp/connection/TcpNioConnectionTests.java