Add "warmup mode" for RPC server.

Start the RPC server before doing all the (expensive) startup
initialisations like loading the block index.  Until the node is ready,
return all calls immediately with a new error signalling "in warmup"
with an appropriate status message (similar to the init message).

This is useful for RPC clients to know that the server is there (e. g.,
they don't have to start it) but not yet available.  It is used in
Namecoin and Huntercoin already for some time, and there exists a UI
hooked onto the RPC interface that actively uses this to its advantage.

1 files changed, 11 insertions, 0 deletions

diff --git a/doc/release-notes.md b/doc/release-notes.md
index 169ad71a0f..6aaea67790 100644
--- a/doc/release-notes.md
+++ b/doc/release-notes.md
@@ -84,3 +84,14 @@ Using wildcards will result in the rule being rejected with the following error
 
     Error: Invalid -rpcallowip subnet specification: *. Valid are a single IP (e.g. 1.2.3.4), a network/netmask (e.g. 1.2.3.4/255.255.255.0) or a network/CIDR (e.g. 1.2.3.4/24).
 
+RPC Server "Warm-Up" Mode
+=========================
+
+The RPC server is started earlier now, before most of the expensive
+intialisations like loading the block index.  It is available now almost
+immediately after starting the process.  However, until all initialisations
+are done, it always returns an immediate error with code -28 to all calls.
+
+This new behaviour can be useful for clients to know that a server is already
+started and will be available soon (for instance, so that they do not
+have to start it themselves).