Allow shared usage of Transaction

[schmidt-sebastian]

This PR:

• Removes the need to round-trip through RemoteStore to create a Transaction.
• Removes the SortedMap dependency in Transaction to reduce code size in the Lite SDK.

[google-oss-bot]

Binary Size Report

Affected SDKs

• @firebase/firestore

  Type	Base (230cd48)	Head (7541784)	Diff
  browser	251 kB	251 kB	+61 B (+0.0%)
  esm2017	195 kB	195 kB	+69 B (+0.0%)
  main	493 kB	494 kB	+123 B (+0.0%)
  module	249 kB	249 kB	+61 B (+0.0%)

• @firebase/firestore/memory

  Type	Base (230cd48)	Head (7541784)	Diff
  browser	192 kB	192 kB	+52 B (+0.0%)
  esm2017	149 kB	149 kB	+60 B (+0.0%)
  main	369 kB	370 kB	+89 B (+0.0%)
  module	190 kB	190 kB	+52 B (+0.0%)

• firebase

  Type	Base (230cd48)	Head (7541784)	Diff
  firebase-firestore.js	290 kB	290 kB	+63 B (+0.0%)
  firebase-firestore.memory.js	232 kB	232 kB	+54 B (+0.0%)
  firebase.js	823 kB	823 kB	+63 B (+0.0%)

Test Logs

• Base (230cd489): https://github.com/firebase/firebase-js-sdk/actions/runs/116206852
• Head (7541784e): https://github.com/firebase/firebase-js-sdk/actions/runs/116536269