This PR refactors how contexts are retained in the managedwriter
package.  Prior to multiplexing, each writer had a dedicated connection,
and context could be shared between writer and the connection.

Now, the relationship between writers and their backing connections is
more nuanced.  Now, we use the context provided as part of client
instantiation (e.g. NewClient) for connection and pool management, and
context provided in NewManagedWriter is scoped to _just_ that specific
writer.

This corrects the previous problem where the context for the first
writer in a connection pool would be used for pool and connection
management, and thus a cancellation of a single  writer's context
could trigger writes from other writer instances to fail.