Refactor migration to support multi chunker

[morgo]

A Pull Request should be associated with an Issue.

We wish to have discussions in Issues. A single issue may be targeted by multiple PRs.
If you're offering a new feature or fixing anything, we'd like to know beforehand in Issues,
and potentially we'll be able to point development in a particular direction.
Further notes in https://github.com/block/spirit/blob/main/.github/CONTRIBUTING.md

Fixes #394
Fixes #393
Fixes #435

The multi path does not yet support checkpoints, checksum, resuming, sentinel tables. There are no tests for multi-path yet: it is working end to end though:

mtocker@BLKD5234QXY40 spirit % go run . --statement="ALTER TABLE bigtable ENGINE=innodb; ALTER TABLE bigtable2 ENGINE=innodb;" --multi --threads=8 --target-chunk-time=5s
INFO[0000] Starting spirit migration: concurrency=8 target-chunk-size=5s
INFO[0000] could not resume from checkpoint: reason=resume-from-checkpoint is not yet supported in multi-statement migrations
INFO[0000] create BinlogSyncer with config {ServerID:1149 Flavor:mysql Host:127.0.0.1 Port:3306 User:spirit Password: Localhost: Charset: SemiSyncEnabled:false RawModeEnabled:false TLSConfig:<nil> ParseTime:false TimestampStringLocation:UTC UseDecimal:false RecvBufferSize:0 HeartbeatPeriod:0s ReadTimeout:0s MaxReconnectAttempts:0 DisableRetrySync:false VerifyChecksum:false DumpCommandFlag:0 Option:<nil> Logger:0x1400028b580 Dialer:0x1007de8f0 RowsEventDecodeFunc:<nil> TableMapOptionalMetaDecodeFunc:<nil> DiscardGTIDSet:false EventCacheCount:10240}
INFO[0000] begin to sync binlog from position (BLKD5234QXY40-bin.015432, 682188214)
INFO[0030] migration status: state=copyRows copy-progress=7545795/40094411 18.82% binlog-deltas=0 total-time=30s copier-time=30s copier-remaining-time=TBD copier-is-throttled=false conns-in-use=9
INFO[0030] finished periodic flush of binary log: total-duration=1.527833ms batch-size=1000

INFO[0060] finished periodic flush of binary log: total-duration=1.158083ms batch-size=1000
INFO[0060] migration status: state=copyRows copy-progress=14320591/40094411 35.72% binlog-deltas=0 total-time=1m0s copier-time=1m0s copier-remaining-time=2m43s copier-is-throttled=false conns-in-use=8
WARN[0067] dynamic chunking is not working as expected: target-time=5s p90-time=1.924584ms new-target-rows=259796402 max-dynamic-row-size=100000
WARN[0067] switching to prefetch algorithm
WARN[0067] disabling chunk prefetching: min-val=13731117 max-val=13732618 max-dynamic-row-size=100000
INFO[0090] migration status: state=copyRows copy-progress=21786956/40094411 54.34% binlog-deltas=0 total-time=1m30s copier-time=1m30s copier-remaining-time=2m21s (±0m from 0s ago) copier-is-throttled=false conns-in-use=8
INFO[0090] finished periodic flush of binary log: total-duration=1.569583ms batch-size=1000
INFO[0120] migration status: state=copyRows copy-progress=26471270/40094411 66.02% binlog-deltas=0 total-time=2m0s copier-time=2m0s copier-remaining-time=1m20s (±0m from 1m ago) copier-is-throttled=false conns-in-use=8
INFO[0120] finished periodic flush of binary log: total-duration=1.10875ms batch-size=1000
INFO[0150] migration status: state=copyRows copy-progress=30598135/40094411 76.32% binlog-deltas=0 total-time=2m30s copier-time=2m30s copier-remaining-time=1m2s (±0m from 1m ago) copier-is-throttled=false conns-in-use=9
INFO[0150] finished periodic flush of binary log: total-duration=1.240709ms batch-size=1000
INFO[0180] approaching the end of the table, synchronously updating statistics
INFO[0180] finished periodic flush of binary log: total-duration=1.372917ms batch-size=1000
INFO[0180] migration status: state=copyRows copy-progress=35555319/40094411 88.68% binlog-deltas=0 total-time=3m0s copier-time=3m0s copier-remaining-time=24s (±0m from 2m ago) copier-is-throttled=false conns-in-use=8
INFO[0192] copy rows complete
INFO[0192] waiting to catch up to source position: (BLKD5234QXY40-bin.015458, 300798619), current position is: (BLKD5234QXY40-bin.015458, 243865612)
INFO[0192] Running ANALYZE TABLE
INFO[0192] waiting to catch up to source position: (BLKD5234QXY40-bin.015460, 997), current position is: (BLKD5234QXY40-bin.015460, 997)
WARN[0192] Attempting final cut over operation (attempt 1/5)
WARN[0192] trying to acquire table locks, timeout: 30
WARN[0192] table lock(s) acquired
INFO[0192] waiting to catch up to source position: (BLKD5234QXY40-bin.015461, 197), current position is: (BLKD5234QXY40-bin.015461, 197)
WARN[0192] table lock released
WARN[0192] final cut over operation complete
INFO[0194] successfully dropped old table: _bigtable_old
INFO[0195] successfully dropped old table: _bigtable2_old
INFO[0195] apply complete: instant-ddl=false inplace-ddl=false total-chunks=860 copy-rows-time=3m12s checksum-time=0s total-time=3m15s conns-in-use=0
INFO[0195] syncer is closing...
INFO[0195] kill last connection id 34863
INFO[0195] syncer is closed