ISSUE-1579: Write Ledger Handle doesn't listen to metadata changes

[sijie]

Original Issue: apache#1579

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BUG REPORT

1. Please describe the issue you observed:
   The writer owns the metadata of the current ensemble only.
   Previous ensemble segments of the same ledger can freely be modified
   by the replication worker.

In the current code, write ledger handle, which allows reads also
are blindsided by the non-disruptive ensemble changes performed
by the replication worker. This could potentially direct readers
to a wrong destination, leading to unsuccessful reads.

Fix this problem by placing a watcher on the zk node just like
readOnlyLedgerHandle. When new metadata is received take the
older (non-current) ensemble segment information and version
number from the new metadata.
BP

This is the master ticket for tracking BP-<bp-number> :

[a short description for this BP]

Proposal PR - #<pr-number>