Fix semantics of Lua calls to SELECT.

Lua scripts are executed in the context of the currently selected
database (as selected by the caller of the script).

However Lua scripts are also free to use the SELECT command in order to
affect other DBs. When SELECT is called frm Lua, the old behavior, before
this commit, was to automatically set the Lua caller selected DB to the
last DB selected by Lua. See for example the following sequence of
commands:

    SELECT 0
    SET x 10
    EVAL "redis.call('select','1')" 0
    SET x 20

Before this commit after the execution of this sequence of commands,
we'll have x=10 in DB 0, and x=20 in DB 1.

Because of the problem above, there was a bug affecting replication of
Lua scripts, because of the actual implementation of replication. It was
possible to fix the implementation of Lua scripts in order to fix the
issue, but looking closely, the bug is the consequence of the behavior
of Lua ability to set the caller's DB.

Under the old semantics, a script selecting a different DB, has no simple
ways to restore the state and select back the previously selected DB.
Moreover the script auhtor must remember that the restore is needed,
otherwise the new commands executed by the caller, will be executed in
the context of a different DB.

So this commit fixes both the replication issue, and this hard-to-use
semantics, by removing the ability of Lua, after the script execution,
to force the caller to switch to the DB selected by the Lua script.

The new behavior of the previous sequence of commadns is to just set
X=20 in DB 0. However Lua scripts are still capable of writing / reading
from different DBs if needed.

WARNING: This is a semantical change that will break programs that are
conceived to select the client selected DB via Lua scripts.

This fixes issue #1811.

src/scripting.c

@@ -994,7 +994,6 @@ void evalGenericCommand(redisClient *c, int evalsha) {
                           readQueryFromClient,c);
     }
     server.lua_caller = NULL;
-    selectDb(c,server.lua_client->db->id); /* set DB ID from Lua client */
 
     /* Call the Lua garbage collector from time to time to avoid a
      * full cycle performed by Lua, which adds too latency.

tests/unit/scripting.tcl

@@ -110,17 +110,21 @@ start_server {tags {"scripting"}} {
         } 0
     } {boolean 1}
 
-    test {EVAL - Is Lua affecting the currently selected DB?} {
+    test {EVAL - Is the Lua client using the currently selected DB?} {
         r set mykey "this is DB 9"
         r select 10
         r set mykey "this is DB 10"
         r eval {return redis.pcall('get','mykey')} 0
     } {this is DB 10}
 
-    test {EVAL - Is Lua seleced DB retained?} {
+    test {EVAL - SELECT inside Lua should not affect the caller} {
+        # here we DB 10 is selected
+        r set mykey "original value"
         r eval {return redis.pcall('select','9')} 0
-        r get mykey
-    } {this is DB 9}
+        set res [r get mykey]
+        r select 9
+        set res
+    } {original value}
 
     if 0 {
         test {EVAL - Script can't run more than configured time limit} {