Implement a connection management mechanism to Spider to make the connection handing easy.

• Each thread gets/returns connections from/to the connection manager. Allocation and deallocation of connections should be done inside the connection manager code.
• Each connection got from the manager should solely belong to a single thread. Note that, in the current implementation, a single connection is referenced by both ha_spider::conns and SPIDER_TRX::trx_conn_hash and this results in the heap-use-after-free (see linked issues) in many test cases.
• Spider 10.3-10.9 has two types of connections. One is for SQL and the other is for handler statements. The latter one has been deprecated from 10.7 and deleted by 10.10. So, we only implement the connection manager for SQL connections.
• For the ease of the development, I recommend creating a patch for 10.10 first and then backporting it to 10.3.