Summary:
This makes max pool size supported in connection manager configurable via gflag `ysql_conn_mgr_max_pool_size` having default value 10000. Earlier this value was hard-coded as 10000 in connection manager.

More details:
   - Make database oid entry and role oid entry list dynamically initialised according to gflag value.
   - Make stats entries list dynamically initialised according to gflag value.

Test Plan: Jenkins: enable connection manager, all tests

Reviewers: rbarigidad, mkumar, skumar, stiwary, vpatibandla

Reviewed By: rbarigidad

Subscribers: ybase, yql

Differential Revision: https://phorge.dev.yugabyte.com/D41360