res_odbc: Implement a connection pool.

Testing has shown that our usage of UnixODBC is problematic
due to bugs within UnixODBC itself as well as the heavy weight
cost of connecting and disconnecting database connections, even
when pooling is enabled.

For users of UnixODBC 2.3.1 and earlier crashes would occur due
to insufficient protection of the disconnect operation. This was
fixed in UnixODBC 2.3.2 and above.

For users of UnixODBC 2.3.3 and higher a slow-down would occur
under heavy database use due to repeated connection establishment.
A regression is present where on each connection the database
configuration is cached again, with the cache growing out of
control.

The connection pool implementation present in this change helps
to mitigate these issues by reducing how much we connect and
disconnect database connections. We also solve the issue of
crashes under UnixODBC 2.3.1 by defaulting the maximum number of
connections to 1, returning us to the previous working behavior.
For users who may have a fixed version the maximum concurrent
connection limit can be increased helping with performance.

The connection pool works by keeping a list of active connections.
If the connection limit has not been reached a new connection is
established. If the connection limit has been reached then the
request waits until a connection becomes available before
continuing.

ASTERISK-26074 #close
ASTERISK-26054 #close

Change-Id: I6774bf4bac49a0b30242c76a09c403d2e856ecff

CHANGES

@@ -381,6 +381,12 @@ res_hep
    valid value using the specified 'uuid_type', the module may fallback to a
    more readily available source for the correlation UUID.
 
+res_odbc
+------------------
+ * A new option has been added, 'max_connections', which sets the maximum number
+   of concurrent connections to the database. This option defaults to 1 which
+   returns the behavior to that of Asterisk 13.7 and prior.
+
 app_confbridge
 ------------------
  * Added a bridge profile option called regcontext that allows you to