[YSQL] Move YBCPgSession and PgTxnManager into PgApiImpl

[d-uspenskiy]

In context of #2934 aggressive buffering of write operation should be implemented.
Buffered operations will be stored in YBCPgSession object.
Buffering should also take transaction state into account. For example before committing transaction all buffered operations must be apply first, and in case transaction is rolled back buffered operations must be vanished.
To control write operation buffer state in YBCPgSession the following redesign should be implemented:

• make YBCPgSession private member of PgApiImpl class
• make PgTxnManager private member of PgApiImpl class
• do not provide direct access to YBCPgSession PgTxnManager
• create separate methods in PgApiImpl class to begin/commit/rollback transactions

With such approach PgApiImpl class will be able to clear/flush buffer of write operation in YBCPgSession object in case of transaction state changes.