Add socket reserve/use connection affinity + fd_version primitives

[rajvarun77]

Generic Controller/Socket plumbing that lets a protocol pin a pooled/short connection across multiple RPCs (connection affinity). This is the shared substrate extracted from the MySQL client PR #3330 so it can be reviewed independently; #3330 is now stacked on top of this branch and contains only the MySQL-specific code.

Controller

• BindSockAction {NONE,RESERVE,USE} encoded in two Controller::_flags bits (FLAGS_BIND_SOCK_RESERVE/USE) via set_bind_sock_action()/bind_sock_action() — no dedicated member added to the hot Controller.
• _bind_sock holds a socket reserved by a previous RPC. EndRPC propagates the action to the current Call; Call::OnComplete reserves the socket (on success) instead of returning it to the pool / failing it; IssueRPC reuses the reserved socket when the action is USE.
• _session_data: opaque per-RPC slot a protocol codec may use to carry typed state between serialize/pack/parse (not owned by Controller).

Socket

• _fd_version (relaxed atomic, bumped on every ResetFileDescriptor) + fd_version() accessor — an ABA guard so an affinity holder can detect that a reserved fd was reconnected underneath it. Atomic because it is written on the reconnect path (no socket lock held) and read from other threads.
• Socket::Write accepts an empty buffer when WriteOptions::auth_flags is set, for server-greets-first handshakes that send their first bytes from the connection-phase handler rather than from data.

Backup/retry fix

Also fixes an uninitialized read this machinery would otherwise introduce: the Call(Call*) copy constructor used for backup requests and retries did not initialize bind_sock_action, so OnComplete could read indeterminate bits and (when they matched RESERVE/USE) divert the backup call's socket away from the pool-return path, hanging the RPC (manifested as ChannelTest.backup_request timing out). Initialized to BIND_SOCK_NONE, matching Call::Reset(); a backup/retry never inherits affinity.

No protocol uses these primitives in this PR; the first consumer is MySQL (#3330).

🤖 Generated with Claude Code

[rajvarun77]

This is the shared-core substrate extracted from #3330 (MySQL client protocol), which is now stacked on top of this PR. Reviewing this first keeps the generic Controller/Socket change small and self-contained.

[rajvarun77]

Reverting the split — folding the backup/retry copy-constructor fix back into #3330 directly; a separate core PR isn't warranted for a one-line initialization fix.