Fix epoch comparison by find_keydir_entry() when keydir->pending != NULL

cc: @engelsanchez 

When the keydir is truly frozen, i.e., when keydir->pending != NULL,
then the comparison of the desired epoch must be greater-than-or-equal.
This fix matches the epoch comparison that is performed by the
`while (s != NULL)` loop in `proxy_kd_entry_at_epoch()`.

I originally found this bug using a merge of the
`bug/deferred-delete-Cd8origin` and `bugfix/fold-open-delete-race`
branches at commits ac71627fb8 and c8c0accc9e5, respectively.

    Cn4 = [[{set,{var,1},{call,bitcask_pulse,fork,[[{init,{state,undefined,false,false,[]}},{set,{not_var,9},{not_call,bitcask_pulse,bc_open,[false,{true,{102,47,431},15}]}},{set,{not_var,15},{not_call,bitcask_pulse,fold,[{not_var,9}]}},{set,{not_var,16},{not_call,bitcask_pulse,fold,[{not_var,9}]}}]]}},{set,{var,2},{call,bitcask_pulse,bc_open,[true,{true,{351,137,29},75}]}},{set,{var,4},{call,bitcask_pulse,put,[{var,2},1,<<51,170,22,66,176,75,118>>]}},{set,{var,5},{call,bitcask_pulse,bc_close,[{var,2}]}},{set,{var,6},{call,bitcask_pulse,fork,[[{init,{state,undefined,false,false,[]}},{set,{not_var,14},{not_call,bitcask_pulse,bc_open,[false,{true,{133,280,435},32}]}},{set,{not_var,26},{not_call,bitcask_pulse,fold,[{not_var,14}]}},{set,{not_var,27},{not_call,bitcask_pulse,fold,[{not_var,14}]}}]]}},{set,{var,7},{call,bitcask_pulse,bc_open,[true,{true,{430,377,266},86}]}},{set,{var,8},{call,bitcask_pulse,merge,[{var,7}]}},{set,{var,18},{call,bitcask_pulse,put,[{var,7},2,<<0,0,0,0,0,0,0,0,0>>]}},{set,{var,20},{call,bitcask_pulse,merge,[{var,7}]}},{set,{var,23},{call,bitcask_pulse,fold_keys,[{var,7}]}},{set,{var,30},{call,bitcask_pulse,merge,[{var,7}]}}],{61218,16404,17227},[{events,[]}]].
    [begin Now = {1404,524363,267720}, io:format("Now ~p ", [Now]), true = eqc:check(eqc:testing_time(24*3600, bitcask_pulse:prop_pulse()), [lists:nth(1, Cn4), Now, lists:nth(3, Cn4)]) end || _ <- lists:seq(1,5)].
    [begin Now = now(), io:format("Now ~p ", [Now]), true = eqc:check(eqc:testing_time(24*3600, bitcask_pulse:prop_pulse()), [lists:nth(1, Cn4), Now, lists:nth(3, Cn4)]) end || _ <- lists:seq(1,550)].

... which results in a fold by the 3rd pid (i.e. the 2nd forked pid)
to find only 1 of the two keys: `<<"kk01">>` is wrongly skipped by the
fold.

A merge prior to the start of the bad fold takes place during another
fold.  That merge copies the only key, `<<"kk01">>`, forward during
epoch 9 ... but that mutation causes the keydir to freeze, so now
keydir->pending != NULL.  When the 3rd pid performs its fold, it is
also in epoch 9.  When folding over the new file, the bug uses a
strictly greater than comparison, so the correct entry in the pending
part of the keydir is skipped, and so fold's get() finds the
old/prior-to-merge entry in the keydir ... and that old entry
does not correspond to the the current fold file and so is skipped.
The fold (correctly) never opens the old/prior-to-merge file, so the
`<<"kk01">>` never appears in the fold's results.