Misc. documentation fixes.

Silences some warnings from Sphinx.
Teach configure script --enable-jemalloc, --with-jemalloc.

Addresses BIT-1128.
Fix --with-jemalloc and make it imply --enable-jemalloc.
Improve DBG_LOG macro (perf. improvement for --enable-debug mode).

Many usages of this macro do some string formatting work inline in the
arguments that end up being unnecessary because the debug stream is
disabled.
Merge remote-tracking branch 'origin/fastpath'

* origin/fastpath:
  Improve DBG_LOG macro (perf. improvement for --enable-debug mode).
  Misc. documentation fixes.
Merge remote-tracking branch 'origin/topic/jsiwek/jemalloc'

* origin/topic/jsiwek/jemalloc:
  Fix --with-jemalloc and make it imply --enable-jemalloc.
  Teach configure script --enable-jemalloc, --with-jemalloc.

BIT-1128 #merged
Updating submodule(s).

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