connection: share handshake ack parsing and account merge

[tradatious]

Description

The blocking and async connections each had their own copy of two parts of the startup handshake
that do not depend on the runtime:

• turning the handshake reply into the server version, connection time and time zone, including
  mapping a reply that ends in UnexpectedEof to Error::ConnectionRejected;
• merging the account-info frames received at startup and copying the result into
  ConnectionMetadata.

Both now live in connection::common as parse_handshake_ack, AccountInfo::merge and
ConnectionMetadata::apply_account_info, and MAX_ACCOUNT_INFO_ATTEMPTS replaces the two local
MAX_ATTEMPTS constants. Each connection calls them and keeps only its own I/O and locking.

One ordering difference, on both clients: the reply is parsed before the metadata lock is taken,
where it used to be parsed while holding it (on the blocking client a bad reply is therefore
reported ahead of a poisoned lock). Nothing else changes.

Testing

New tests in connection/common_tests.rs: test_parse_handshake_ack (a valid reply, an
UnexpectedEof reply, another I/O error), test_account_info_merge and
test_connection_metadata_apply_account_info. The existing sync and async handshake tests pass
unchanged. cargo fmt --check; clippy, rustdoc (-D warnings) and cargo test on all three feature
configurations; cargo build --examples both ways; integration crates built.

Breaking changes

No. Crate-internal.