Sqlite can end up with valid utf8 sequences which describe invalid codepoints, which break synapse_port_db

This issue has been migrated from [#3538](https://github.com/matrix-org/synapse/issues/3538).

---

Error ends up looking like:
```
2018-07-15 22:54:50,837 - synapse.metrics - 256 - INFO - Collecting gc 0
2018-07-15 22:54:50,936 - synapse_port_db - 562 - ERROR -
Traceback (most recent call last):
File "/usr/bin/synapse_port_db", line 552, in run
consumeErrors=True,
FirstError: FirstError[#16, [Failure instance: Traceback: : invalid byte sequence for encoding "UTF8": 0xed 0xb3 0xb6

/usr/lib/python2.7/dist-packages/twisted/internet/defer.py:434:errback
/usr/lib/python2.7/dist-packages/twisted/internet/defer.py:501:_startRunCallbacks
/usr/lib/python2.7/dist-packages/twisted/internet/defer.py:588:_runCallbacks
/usr/lib/python2.7/dist-packages/twisted/internet/defer.py:1184:gotResult
--- ---
/usr/lib/python2.7/dist-packages/twisted/internet/defer.py:1126:_inlineCallbacks
/usr/lib/python2.7/dist-packages/twisted/python/failure.py:389:throwExceptionIntoGenerator
/usr/bin/synapse_port_db:269:handle_table
/usr/lib/python2.7/dist-packages/twisted/internet/defer.py:1126:_inlineCallbacks
/usr/lib/python2.7/dist-packages/twisted/python/failure.py:389:throwExceptionIntoGenerator
/usr/bin/synapse_port_db:428:handle_search_table
/usr/lib/python2.7/dist-packages/twisted/python/threadpool.py:246:inContext
/usr/lib/python2.7/dist-packages/twisted/python/threadpool.py:262:
/usr/lib/python2.7/dist-packages/twisted/python/context.py:118:callWithContext
/usr/lib/python2.7/dist-packages/twisted/python/context.py:81:callWithContext
/usr/lib/python2.7/dist-packages/twisted/enterprise/adbapi.py:298:_runWithConnection
/usr/bin/synapse_port_db:138:r
/usr/bin/synapse_port_db:415:insert
/usr/lib/python2.7/dist-packages/synapse/storage/_base.py:90:executemany
/usr/lib/python2.7/dist-packages/synapse/storage/_base.py:117:_do_execute
]]
```
in the event_search logic.

Turns out that 0xed 0xb3 0xb6 is valid utf8, but describes \uDCF7 which is not a valid defined codepoint, which postgres barfs on when you try to insert it.

Python2 doesn't recognise there being anything invalid about it, however.

The workaround in the end was to use `iconv_codecs` to use `iconv` to strip invalid codepoints out of the string before handing to postgres, with something like:

`row["value"].encode("iconv:utf8", "ignore").decode("utf8")`

Which seemed to work on linux, but fails on macOS.

Thanks to `@flux:matrix.org` for reporting and debugging this!

The original cause of the bad data is https://github.com/matrix-org/synapse/issues/3537