Optimize couch_util:to_hex/1
When profiling [1] a cluster acting as a replication source, noticed a lot of
time spent in `couch_util:to_hex/1`. That function is used to emit revision ids
amongst other things. When processing a million documents with a 1000 revisions
each, it ends up in the hotpath, so to speak.
Remembering that in Erlang/OTP 24 there is a new `binary:encode_hex/1` [2]
function, decided to benchmark ours vs the OTP implementation. It turns ours is
slower [3] so let's try to use the OTP one.
One difference from the OTP's version is ours emits lower case hex letters,
while the OTP one emits upper case ones. That's why the lookup table had all
the "A"s replaced with "a"s, "B"s with "b"s, etc.
As a bonus, replaced a few calls to `couch_util:to_hex/1` wrapped in `?l2b/1`
or `list_to_binary/1` with just a single call to `couch_util:to_hex_bin/1`.
Existing `couch_util:to_hex/1` version, returning a list ,was left as is and
just calls `to_hex_bin/1` internally and converts the result to a list.
This altered the peak memory usage for the key tree stemming test so had to
alter the magic constants there a bit to avoid flakiness.
[1]
```
> ... eprof:analyze(total,[{sort, time}, {filter, [{time, 1000}, {calls, 100}]}]).
FUNCTION CALLS % TIME [uS / CALLS]
-------- ----- ------- ---- [----------]
...
couch_doc:revid_to_str/1 1165641 1.71 402860 [ 0.35]
couch_key_tree:get_key_leafs_simple/4 1304102 1.85 435700 [ 0.33]
erlang:list_to_integer/2 873172 2.00 471140 [ 0.54]
gen_server:loop/7 62209 2.11 496932 [ 7.99]
couch_key_tree:map_simple/3 1829235 2.36 554429 [ 0.30]
couch_util:nibble_to_hex/1 37334650 16.67 3917127 [ 0.10]
couch_util:to_hex/1 19834192 34.37 8077050 [ 0.41]
--------------------------------------------------------------- -------- ------- -------- [----------]
Total: 91072005 100.00% 23503072 [ 0.26]
```
[2] https://www.erlang.org/doc/man/binary.html#encode_hex-1
[3]
```
% ~/src/erlperf/erlperf 'hex:to_hex1(<<210,90,95,232,68,185,66,248,160,33,184,103,181,221,158,96>>).' 'hex:to_hex3(<<210,90,95,232,68,185,66,248,160,33,184,103,181,221,158,96>>).'
Code || QPS Time Rel
hex:to_hex3(<<210,90,95,232,68,185,66,248,160,33,184,103,181,221,158,96>>). 1 2746 Ki 364 ns 100%
hex:to_hex1(<<210,90,95,232,68,185,66,248,160,33,184,103,181,221,158,96>>). 1 1593 Ki 627 ns 58%
```
(`to_hex1/1` is the existing version and `to_hex3/1` is the OTP version).
