[3.2 -> main] Backport SHiP fixes from eosio/eos release/2.2.x

[heifner]

Backport of state_history_plugin fixes: EOSIO/eos#10608

From the backport:

• Create an additional thread for all ship network communication. This thread already existed in release/3.2 but was not used. See state_history_plugin unused code #593.
• Fix bug where state history plugin would send blocks with block_num smaller than the client requests when the state history block head is behind the block head of the client.

Additional changes:

• Remove session_manager_t and use post to main thread so session_set is only accessed from main thread.
• Use named_thread_pool to simplify SHiP thread. Older versions of boost do not provide an acceptor that takes a strand in constructor, so rely on implicit strand of single thread running io_context which is much simpler than wrapping all calls in boost::asio::bind_executor.
• Made sure the destructor of boost beast web socket stream is called on the SHiP thread. The session tests in Merge SHiP stability fixes & log splitting & memory usage improvements #628 would consistently crash in the the stream destructor when destroyed on the wrong thread.

Reproduced the crash of #592. After this PR, was unable to reproduce #592 crash.

This PR does not include accept too_many_files_open fix: #603
#577 has some of these changes already but takes a slightly different approach. I've updated #577 with the thread separation changes of this PR, see #628.

Resolves #592
Merges #624 (release/3.2) into main. These changes have also been applied in #628 along with other cleanups.