Add interface to generic-pool getters

[j1banez]

If one needs to access pool state (total size, available objects and in use objects) it has to access the genericPool getters like this:

const Pool = require('pg-pool');

const pool = new Pool();

const size = pool.pool.getPoolSize();
const available = pool.pool.availableObjetcsCount();
const borrowed = pool.pool.inUseObjectsCount();

While it works it is a bit risky cause pool.pool is an internal generic-pool 2.4.3 instance thus we have no control to when generic-pool will be updated and when the api will break, and it will break because generic-pool 3's getters has been renamed.

So I suggest with this PR to add pg-pool getters that wrap generic-pool getters in order to safely access the pool state,
When pg-pool will update his version of 'generic-pool' we will just need to update the pg-pool getters
to use the new names of the generic-pool getters and it will be transparent with no breaking changes for the users.

[sibedge]

This PR could be part of it: #59

as it does the same, specifically for one flag.

[brianc]

generic pool has been removed as a dependency, so all the getters are now directly on pool and anything documented is considered part of the public API and wont be changing between major versions. So...yay! 💃

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