Remove MultiTabSyncEngine

[schmidt-sebastian]

This is similar to #3436 and removes the explicit MultiTabSyncEngine class. Instead, the methods are now tree-shakeable and will only be included with multi-tab IndexedDb persistence.

Much smaller diff: https://github.com/firebase/firebase-js-sdk/pull/3442/files?w=1

This is the second commit in #3425. The next PR will then rewrite the component provider.

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Latest commit: a70479b

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Binary Size Report

Affected SDKs

• @firebase/firestore

  Type	Base (7d5678b)	Head (2a2eca4)	Diff
  browser	246 kB	246 kB	-502 B (-0.2%)
  esm2017	193 kB	193 kB	-75 B (-0.0%)
  main	469 kB	469 kB	+5 B (+0.0%)
  module	244 kB	244 kB	-492 B (-0.2%)
  react-native	194 kB	193 kB	-75 B (-0.0%)

• @firebase/firestore/exp

  Type	Base (7d5678b)	Head (2a2eca4)	Diff
  browser	187 kB	187 kB	-75 B (-0.0%)
  main	508 kB	508 kB	-10 B (-0.0%)
  react-native	187 kB	187 kB	-75 B (-0.0%)

• @firebase/firestore/lite

  Type	Base (7d5678b)	Head (2a2eca4)	Diff
  main	494 kB	494 kB	-10 B (-0.0%)

• @firebase/firestore/memory

  Type	Base (7d5678b)	Head (2a2eca4)	Diff
  browser	185 kB	185 kB	+93 B (+0.1%)
  esm2017	145 kB	145 kB	+103 B (+0.1%)
  main	344 kB	344 kB	+279 B (+0.1%)
  module	183 kB	183 kB	+93 B (+0.1%)
  react-native	145 kB	145 kB	+103 B (+0.1%)

• firebase

  Type	Base (7d5678b)	Head (2a2eca4)	Diff
  firebase-firestore.js	285 kB	284 kB	-525 B (-0.2%)
  firebase-firestore.memory.js	224 kB	224 kB	+86 B (+0.0%)
  firebase.js	820 kB	819 kB	-526 B (-0.1%)

Test Logs

• Base (7d5678be): https://github.com/firebase/firebase-js-sdk/actions/runs/176264466
• Head (2a2eca43): https://github.com/firebase/firebase-js-sdk/actions/runs/176360650

[schmidt-sebastian]

Note: This combines the MultiTabSyncEngine's applyOnlineStateChange with the SyncEngine's applyOnlineStateChange. It is actually similar to what we had 6+ months ago.

[schmidt-sebastian]

This field is now only part of WebStorageSharedClientState. Removing it made the general test setup a lot easier, since it is not accessed in most tests but would still have to be assigned.