There is no need to depend on the old context package now that the
minimum Go version is 1.7. The move to "context" eliminates our weird
vendoring setup. Some vendored code still uses golang.org/x/net/context
and it is now vendored in the normal way.
This change triggered new vet checks around context.WithTimeout which
didn't fire with golang.org/x/net/context.
If a batch request contained an invalid method, the server would reply
with a non-batch error response. Fix this by tracking an error for each
batch element.
The server delayed closing of connections for 3s when stopping. This was
supposed to allow for slow handlers, but it didn't really work. When
geth quits, it will just exit immediately after quitting the server.
Removing the timer makes testing easier because all connections will be
closed after Stop returns.
Context keys must have a unique type in order to prevent
any unintented clashes. The code used int(1) as key.
Fix it by implementing the pattern recommended by package context.
