Connection pooling support using 'pg' module's connection pooling 

Added connection pooling support using 'pg' module's in-build connection pooling mechanism

Schema definition contains an additional parameter poolSize that is passed to 'pg' module.
As far as I understand it will try connect before each query. It is not clear whether it will create new connection each time or reuse existing connection. If it creates new connection - this is not acceptable. If not - can't we just call it once and keep `client` instance on adapter to be able reuse it without adding overhead for calling pg.connect each time?

And another thing - indentation (should be 4 spaces) and style of spacing (should be `if (cond` and not `if( cond`. Same for function call: should be `pg.connect(this.settings, ...`