Merge #15691: 0.18: rc3 backports

95faffed qa: Check unconfirmed balance after loadwallet (João Barbosa)
59716ec3 wallet: Update transactions with current mempool after load (João Barbosa)
ed0498af interfaces: Add Chain::requestMempoolTransactions (João Barbosa)
ebf65666 wallet: Move CWallet::ReacceptWalletTransactions locks to callers (João Barbosa)
a90db2f1 [tests] Add test for wallet rebroadcasts (John Newbery)
50c56f2f Interrupt orphan processing after every transaction (Pieter Wuille)
bb60121d [MOVEONLY] Move processing of orphan queue to ProcessOrphanTx (Pieter Wuille)
6355214f Simplify orphan processing in preparation for interruptibility (Pieter Wuille)

Pull request description:

  Remaining backports for rc3

ACKs for commit 95faff:
  promag:
    ACK 95faffed, well done in ed0498af - verified all cherry picks.

Tree-SHA512: 597ee45493ac04e8c3149628a357c3238169d4b4f78158f22a3531e5f66289c228ecd6582f99026698883216f1ee7934d6315d19c99fc5f4f33dd1bed4300186