erts: Fix memory leak for distributed monitors
xmerl: Add tests for XPath queries that resolve the context namespace

Ensures that both the original namespace prefix and a namespace prefix
provided to the xmlContext will both return the same sets of nodes.
xmerl: Look up unknown prefixes in xmlContext when matching attributes

The core use case is a query where the original prefix in the scanned
document is unknown (or varying).  For example:

  xmerl_xpath:scan("//@ns:name", Doc, [{namespace, [{"ns", Uri}]}])

Previously, this would only return a result if the namespace prefix
was an exact match.
xmerl: Use context namespace declarations to resolve prefix node tests

Previously, a match would not be found if the namespace prefix in the
XPath query was not contained in the original document.  This allows
the `namespace' option to provide a prefix that will be resolved to a
namespace URI.

See Section 2.3 of the XPath 1.0 specification for the behaviour of
'NCName:*' node tests.
Merge branch 'sverk/monitor-memory-leak/OTP-11410' into maint

* sverk/monitor-memory-leak/OTP-11410:
  erts: Fix memory leak for distributed monitors
Merge branch 'danielwhite/xmerl-xpath-resolve-context-namespaces/OTP-11461' into maint

* danielwhite/xmerl-xpath-resolve-context-namespaces/OTP-11461:
  xmerl: Use context namespace declarations to resolve prefix node tests
  xmerl: Look up unknown prefixes in xmlContext when matching attributes
  xmerl: Add tests for XPath queries that resolve the context namespace