Support cluster discovery in MeshConnection

This feature adds the new optional arguments to the MeshConnection
contructor:

* `cluster_discovery_function` -- a name of the function which will be
  periodically called on a currently connected tarantool instance to
  update a list of MeshConnection addresses.
* `cluster_discovery_delay` -- minimal amount of seconds between address
  list updates (default is 60 seconds).

The update of addresses is performed right after successful connecting
and before performing a request (if a minimal time passes).

This commits changes the round robin retry strategy. Before it performs
two attempts to connect to each address reconnect_max_attempts times (3
by default), now it do that only once.

The new type of error is added: ConfigurationError. It is risen when a
user provides incorrect configuration: say, one of provided addresses is
not correct.

The new type of warning is added: ClusterDiscoveryWarning. This warning
is shown when a something went wrong during cluster discovery: say, one
of returned addresses is not correct. Note the difference: a user
provided configuration verified strictly, while a cluster discovery
function result is filtered (with warnings) and good addresses are
applied (if the list is not empty).

Aside of the new functionality this commit improves compatibility of
MeshConnection API with Connection. The following arguments are added to
the MeshConnection constructor: `host`, `port`, `call_16`,
`connection_timeout`. An address from `host` / `port` arguments is added
to `addrs` (if provided) as the first item.

Fixes #134.

Author: denis-ignatenko

doc/api/class-mesh-connection.rst

@@ -0,0 +1,8 @@
+
+.. currentmodule:: tarantool.mesh_connection
+
+class :class:`MeshConnection`
+-----------------------------
+
+.. autoclass:: MeshConnection
+

doc/index.rst

@@ -40,6 +40,7 @@ API Reference
 
    api/module-tarantool.rst
    api/class-connection.rst
+   api/class-mesh-connection.rst
    api/class-space.rst
    api/class-response.rst
 

doc/index.ru.rst

@@ -40,6 +40,7 @@
 
    api/module-tarantool.rst
    api/class-connection.rst
+   api/class-mesh-connection.rst
    api/class-space.rst
    api/class-response.rst
 

tarantool/const.py

@@ -86,3 +86,5 @@
 RECONNECT_MAX_ATTEMPTS = 10
 # Default delay between attempts to reconnect (seconds)
 RECONNECT_DELAY = 0.1
+# Default cluster nodes list refresh interval (seconds)
+CLUSTER_DISCOVERY_DELAY = 60

tarantool/error.py

@@ -43,6 +43,12 @@ class InterfaceError(Error):
     '''
 
 
+class ConfigurationError(Error):
+    '''
+    Error of initialization with a user-provided configuration.
+    '''
+
+
 # Monkey patch os.strerror for win32
 if sys.platform == "win32":
     # Windows Sockets Error Codes (not all, but related on network errors)
@@ -152,6 +158,11 @@ class NetworkWarning(UserWarning):
     pass
 
 
+class ClusterDiscoveryWarning(UserWarning):
+    '''Warning related to cluster discovery'''
+    pass
+
+
 # always print this warnings
 warnings.filterwarnings("always", category=NetworkWarning)
 
@@ -166,6 +177,7 @@ def warn(message, warning_class):
     line_no = frame.f_lineno
     warnings.warn_explicit(message, warning_class, module_name, line_no)
 
+
 _strerror = {
     0: ("ER_UNKNOWN", "Unknown error"),
     1: ("ER_ILLEGAL_PARAMS", "Illegal parameters, %s"),