Add embedded driver retries and reopen-on-contention for multi-process use

[coffeegoddd]

This PR makes the embedded Dolt database/sql driver resilient to multi-process contention by adding configurable retries with backoff and ensuring retries reopen
the underlying Dolt SQL engine so connections don’t get stuck in a read-only/fallback state.

Key changes

• Retry policy via DSN params: retry, retrytimeout, retrymaxattempts, retryinitialdelay, retrymaxdelay.
• Embedded-mode tuning DSN params:
• nocache=true to disable Dolt’s in-process singleton DB cache during embedded opens
• failonlocktimeout=true to fail fast on journal lock timeout (avoids silent read-only fallback)
• Retry logic:
• Retries now apply to engine open (so Ping() / first connect can succeed after contention clears)
• Retries apply to statement execution (reopen engine + backoff + retry on retryable errors)
• Engine reopen support:
• DoltConn stores datasource + retry policy, uses a mutex, can rebuild se + gmsCtx
• Statements/transactions reference the connection and always fetch the current engine/context
• Error classification:
• Adds conservative retryable detection (including Dolt’s new nbs.ErrDatabaseLocked).
• Tests:
• New multi-process contention tests (concurrency_lock_test.go) covering:
• no-retry fast failure under contention
• retry-enabled eventual success after lock release