add max and min pool idle config for sql sinks

[AlvaroVega]

Related #2366

doc about GenericObjectPool
https://commons.apache.org/proper/commons-pool/apidocs/org/apache/commons/pool2/impl/GenericObjectPool.html

$ psql -h localhost -U postgres 
SELECT datname, count(*) FROM pg_stat_activity
  WHERE state = 'idle' GROUP BY datname;

This PR allows change:

• maxPoolSize
• maxPoolIdle
• minPoolIdle
• minPoolIdleTimeMillis

And fixes default initial values for:

• maxPoolSize: 3
• maxPoolIdle: 2
• minPoolIdle: 0
• minIddleTimeMillis: 10000

With the following meanings:
// Sets the cap on the number of objects that can be allocated by the pool (checked out to clients, or idle awaiting checkout) at a given time.
gPool.setMaxActive(this.maxPoolSize);

// Sets the cap on the number of "idle" instances in the pool.
gPool.setMaxIdle(this.maxPoolIdle);

// Sets the minimum number of objects allowed in the pool before the evictor thread (if active) spawns new objects.
gPool.setMinIdle(this.minPoolIdle);

// Sets the minimum amount of time an object may sit idle in the pool before it is eligible for eviction by the idle object evictor (if any)
gPool.setMinEvictableIdleTimeMillis(this.minPoolIdleTimeMillis);

// Sets the number of milliseconds to sleep between runs of the idle object evictor thread
gPool.setTimeBetweenEvictionRunsMillis(this.minPoolIdleTimeMillis*3)

• Upgrade doc with these new config entries: https://github.com/telefonicaid/fiware-cygnus/blob/master/doc/cygnus-ngsi/flume_extensions_catalogue/ngsi_postgresql_sink.md#section2.

how to see number of idle connections:

[AlvaroVega]

This is a example about how now cygnus uses a connection, then changes to idle and finally is released

[fgalan]

LGTM