Docs: add a design document for the Client

Add a document that states the requirements and use cases we have in mind
for the Client, and then proposes a design that fulfills them.

docs/client-design.md

@@ -0,0 +1,74 @@
+# Tntcxx Client Design
+
+## Scope
+
+This document describes the design of the Client part of Tntcxx. First we 
+state the requirements and use cases we have in mind, and then we propose a 
+design that fulfills them.
+
+## Requirements
+
+### Functional Requirements
+
+We envision Tntcxx to be used primarily as a part of network applications (e.g.,
+an HTTP server), the Client backing requests to Tarantool. Such applications are
+usually built around a central event processing loop, which is responsible for 
+multiplexing network I/O.
+
+#### Application Event Processing Loop Integration
+
+The first and foremost requirement is that there must be a convenient 
+way to integrate the Tntcxx Client into the event processing loop used by the 
+application. Moreover, the Client must never run the event processing 
+loop itself.
+
+At the same time, since event processing loops are inherently single threaded, 
+we do expect the Tntcxx Client to be used in a multithreaded environment, i.e., 
+when connections and reqeust futures are in different threads. So we do not aim
+to design the Client to be thread-safe.
+
+#### Asynchronous Request Processing
+
+Since the Tntcxx Client is constrained from running the event processing loop, 
+the Client must support asynchronous request processing through 
+application-provided callbacks. The Client must only be responsible for 
+posting requests — the responses must be processed and returned to the 
+application as they arrive, i.e., asynchronously.
+
+#### Connection Status
+
+The application must be able to query the status of a Tntcxx Client.
+
+#### Request Readiness
+
+The application must be able to query the readiness of a request issued 
+through the Tntcxx Client.
+
+#### Request Cancelling
+
+The application must be able to cancel a request issued through the Tntcxx 
+Client.
+
+#### Error Handling
+
+There must be a convenient exception-safe way for the application to handle 
+errors arising throughout the Tntcxx Client request lifecycle.
+
+#### Should the Request Handle give access to the response?
+
+#### NetworkProvider: Proactor vs Reactor?
+
+#### Request Retrying?
+
+#### Transactions?
+
+#### Connection Pool?
+
+#### Failover?
+
+### Non-Functional Requirements
+
+#### System Call Minimization
+
+#### Scalability
+