Add. Support multiple database backends.

```Lua
local Database = require 'resources.functions.database'

-- Default backend configured via xml_handler.db_backend = 'native'
dbh = Database.new("system")

-- To use other backends you can use
dbh = Database.backend.luasql("system")
```

resources/install/scripts/odbc_pool.lua

@@ -0,0 +1,67 @@
+-- Start background service to support Lua-ODBC-Pool database backend
+
+require "resources.functions.config"
+require "resources.functions.file_exists"
+
+local log      = require "resources.functions.log".dbpool
+local odbc     = require "odbc"
+local odbcpool = require "odbc.pool"
+
+-- Configuration
+local POLL_TIMEOUT = 5
+local run_file = scripts_dir .. "/run/dbpool.tmp";
+
+-- Pool ctor
+local function run_odbc_pool(name, n)
+  local connection_string = assert(database[name])
+
+  local typ, dsn, user, password = connection_string:match("^(.-)://(.-):(.-):(.-)$")
+  assert(typ == 'odbc', "unsupported connection string:" .. connection_string)
+
+  local cli = odbcpool.client(name)
+
+  log.noticef("Starting reconnect thread[%s] ...", name)
+  local rthread = odbcpool.reconnect_thread(cli, dsn, user, password)
+  rthread:start()
+  log.noticef("Reconnect thread[%s] started", name)
+
+  local env = odbc.environment()
+
+  local connections = {}
+  for i = 1, (n or 10) do
+    local cnn = odbc.assert(env:connection())
+    connections[#connections+1] = cnn
+    cli:reconnect(cnn)
+  end
+
+  return {
+    name = name;
+    cli  = cli;
+    cnn  = connections;
+    thr  = rthread;
+  }
+end
+
+local function stop_odbc_pool(ctx)
+  log.noticef("Stopping reconnect thread[%s] ...", ctx.name)
+  ctx.thr:stop()
+  log.noticef("Reconnect thread[%s] stopped", ctx.name)
+end
+
+local function main()
+  local system_pool = run_odbc_pool("system", 10)
+  local switch_pool = run_odbc_pool("switch", 10)
+
+  local file = assert(io.open(run_file, "w"));
+  file:write("remove this file to stop the script");
+  file:close()
+
+  while file_exists(run_file) do
+    freeswitch.msleep(POLL_TIMEOUT*1000)
+  end
+
+  stop_odbc_pool(system_pool)
+  stop_odbc_pool(switch_pool)
+end
+
+main()