connection-pool: implement connection pool with master discovery

Main features:

- Return available connection from pool according to
  round-robin strategy.
- Automatic master discovery by `mode` parameter.

Additional options (configurable via `ConnectWithOpts`):

* `CheckTimeout` - time interval to check for connection
  timeout and try to switch connection

`Mode` parameter:

* `ANY` (use any instance) - the request can be executed on any
  instance (master or replica).
* `RW` (writeable instance (master)) - the request can only
  be executed on master.
* `RO` (read only instance (replica)) - the request can only
  be executed on replica.
* `PREFER_RO` (prefer read only instance (replica)) - if there is one,
  otherwise fallback to a writeable one (master).
* `PREFER_RW` (prefer write only instance (master)) - if there is one,
  otherwise fallback to a read only one (replica).

Closes #113

Author: AnaNek

CHANGELOG.md

@@ -16,6 +16,7 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
 - Support UUID type in msgpack (#90)
 - Go modules support (#91)
 - queue-utube handling (#85)
+- Master discovery (#113)
 
 ### Fixed
 

Makefile

@@ -14,6 +14,12 @@ deps: clean
 test:
 	go test ./... -v -p 1
 
+.PHONY: test-connection-pool
+test-connection-pool:
+	@echo "Running tests in connection_pool package"
+	go clean -testcache
+	go test ./connection_pool/ -v -p 1
+
 .PHONY: test-multi
 test-multi:
 	@echo "Running tests in multiconnection package"

connection_pool/config.lua

@@ -0,0 +1,35 @@
+-- Do not set listen for now so connector won't be
+-- able to send requests until everything is configured.
+box.cfg{
+    work_dir = os.getenv("TEST_TNT_WORK_DIR"),
+}
+
+box.once("init", function()
+    box.schema.user.create('test', { password = 'test' })
+    box.schema.user.grant('test', 'read,write,execute', 'universe')
+
+    local s = box.schema.space.create('testPool', {
+        id = 520,
+        if_not_exists = true,
+        format = {
+            {name = "key", type = "string"},
+            {name = "value", type = "string"},
+        },
+    })
+    s:create_index('pk', {
+        type = 'tree',
+        parts = {{ field = 1, type = 'string' }},
+        if_not_exists = true
+    })
+end)
+
+local function simple_incr(a)
+    return a + 1
+end
+
+rawset(_G, 'simple_incr', simple_incr)
+
+-- Set listen only when every other thing is configured.
+box.cfg{
+    listen = os.getenv("TEST_TNT_LISTEN"),
+}