Flow: decouple buffer size from various operators and fuse

[elizarov]

There is a number of flow operators that create several coroutines and/or establish channel to communicate between them: flowOn, channelFlow, produceIn, broadcastIn, flattenMerge, flatMapMerge. Buffer size in those operators is an optional an orthogonal concept that is not essential to those operations, so there is a proposal.

Introduce a separate buffer operator that becomes the only operator that takes buffer size parameter (parameter will be named capacity). This operator has its independent use-case -- it decouples upstream flow emitter from the flow collector by running them in different coroutine with a channel between them.

When this buffer operator is adjacent to any of the aforementioned operators it "fuses" with them and configures their buffer size. Note, that some operators like flowOn optionally create a channel (flowOn only creates a channel when there is a need to change a dispatcher), while buffer operator explicitly requests channel creation. The goal of fusion is to create the minimum number of channels -- only when they are required or explicitly requested.

The ultimate use-case of channel operator fusion is demonstrated by the following code:

channelFlow { 
    while(true) { 
        send(computeSomeValue()) 
    } 
}
    .flowOn(Dispatchers.IO)
    .buffer(32)
    .broadcastIn(scope) // terminal op that returns BroadcastChannel

This chain of calls leads to creation of a single BroadcastChannel instance with a buffer size of 32 and a coroutine that calls computeSomeValue() in IO dispatcher that directly sends to this broadcast channel as if broadcast(IO, 32) { ... } builder was used. A truly declarative way to configure the corresponding computation topology.

Adjacent calls to flowOn in any combination with buffer calls are also fused.

As a part of this change the "default" size of the channel is centralized. A special Channel.DEFAULT marker is introduced. It causes Channel(...) and BroadcastChannel(...) factory to builders to create a channel with a default capacity (16 by default). This default is used unless the buffer size is explicitly specified.